Mishnah Kelim is in Seder Taharot and deals with the laws of utensils and their susceptibility to tumah, ritual impurity. Kelim is especially sensitive to a subtle question: when is an object merely material, and when has it become a kli, a vessel? The answer is not only physical. It depends on form, completion, intention, and use.
The Mishnah says:
The covers of wine jugs, oil jugs, and Neyarian barrels are tahor, pure, but if one prepared them for use, they become tamei, susceptible to impurity. The cover of a stewpot, if it has a hole or a protrusion, is tahor; if it lacks those features, it is tamei, because it can be used to strain vegetables into the pot. Rabbi Eliezer bar Tzadok says: because one can overturn the cooked vegetable mass upon it.
A gistera found in a kiln: before its manufacture was completed, it is tahor; after its manufacture was completed, it is tamei. A titros: Rabbi Eliezer bar Tzadok declares it tahor, while Rabbi Yose declares it tamei, because it lets liquid out drop by drop. 
The plain halachic movement is delicate. A cover is not automatically a vessel. A broken-looking piece in a kiln is not automatically a vessel. A perforated object is not automatically useless. The Mishnah asks: has this thing entered the world of function? Can it receive, hold, strain, pour, serve, or be designated by human intention? Once it becomes a kli, it can become tamei. Before it becomes a kli, it remains outside that category.
This is a great yesod in avodat HaShem.
A person may think that tumah attaches to “badness.” But in these Mishnayot, tumah attaches to vesselhood. The more something becomes capable of function, the more it enters responsibility. A stone lying in a field is not the same as a cup on the table. Clay in the earth is not the same as a formed utensil. Potential alone does not yet carry the full burden of sacred accountability. But once form, completion, and intention arrive, the object stands before Torah as something that can be elevated or damaged.
So too with the human being.
When a person is raw clay, confused, unformed, still “in the kiln,” still not fully aware of his task, Heaven judges him differently. But when the person becomes a kli, when he has da’at, capacity, influence, speech, learning, family, money, position, or emotional power, he becomes capable of holding more. And because he can hold more, he must guard more.
The Mishnah speaks about covers. A cover seems secondary. It is not the pot. It is not the wine. It is not the oil. It only covers something else. Yet the Mishnah says that even a cover, if prepared for use, becomes a vessel. This is an enormous teaching. In avodah, even the “secondary” parts of life become vessels when they are consciously used. A commute can become a kli for tefillah. A meal can become a kli for gratitude. A conversation can become a kli for chesed. A difficulty can become a kli for humility. A boundary can become a kli for kedushah.
Nothing remains neutral once it is prepared for service.
This is close to the inner language that holiness descends through measure, order, and readiness. Light cannot be received by chaos. “True light is not chaos, excess, or unbounded exposure,” but light that enters “vessels, measures, distinctions, boundaries, and forms,” so creation can receive without being nullified. 
That is why the Mishnah’s tiny details matter. A hole, a protrusion, a completed firing in the kiln, a designation for use, a capacity to release liquid drop by drop. The Torah is teaching us to look at reality with refined eyes. The difference between tahor and tamei may rest on whether this object has crossed the threshold into functional vesselhood.
In Pnimiyut HaTorah, this opens into the secret of orot and kelim, lights and vessels. Sefer Etz Chaim teaches that the formation of vessels requires contraction and measure, because an unmeasured abundance of light would overwhelm the vessel; only after the vessel is formed can light return in a measured way that the vessel can bear.  Tanya expresses the work of tikkun as bringing increased Divine light into the vessels of the world through Torah and mitzvot, elevating the material rather than indulging it. 
Now we can return to the Mishnah with deeper eyes.
A cover that is not prepared for use is tahor. It has not yet accepted a defined avodah. But once it is prepared, it becomes a kli. This is the soul’s moment of commitment. Before commitment, a talent is only a possibility. After commitment, the talent becomes holy responsibility.
A gistera in the kiln before completion is tahor. The fire is still working. The form is still becoming. This is the person in process. One must not judge the unfinished soul as though it were finished. Many people are still in the kiln of becoming. Their heat, pressure, confusion, and waiting are not signs of rejection. They are part of formation.
But after completion, the same gistera becomes tamei. Why? Because completion brings accountability. A mature vessel can no longer say, “I am only clay.” Once HaShem has shaped a person through Torah, life, suffering, kindness, teachers, parents, children, and repeated hashgacha pratit, the person is no longer allowed to live as unformed matter. He is a vessel.
And the titros, which releases liquid drop by drop, reveals one more layer. There are vessels that do not pour in abundance. They release slowly, almost secretly, drop by drop. Rabbi Yose sees this as enough function to grant it the status of a kli. Spiritually, this is very precious. A person should not say, “Since I cannot pour like a river, I am not a vessel.” Some souls are made to give drop by drop: one kind word, one small act of restraint, one quiet forgiveness, one honest berakhah, one page learned, one coin of tzedakah, one moment of silence instead of anger.
A drop can prove vesselhood.
The avodah of these Mishnayot is therefore to ask: where has HaShem already made me into a vessel? Where am I still in the kiln? Where have I been using something casually that now needs to be consciously prepared for holiness? And where am I dismissing my small drops, when in truth those drops may be the very sign that I am already a kli in the service of HaShem?
The Mishnah is not only cataloging ancient pottery. It is training the Jewish eye to see the border between raw material and holy function. And once a person sees that border, he begins to understand his own life differently.
The body is a vessel. Speech is a vessel. Time is a vessel. Money is a vessel. Pain is a vessel. Memory is a vessel. Even a broken piece, if it can still serve, remains spiritually meaningful.
The highest request is not merely, “HaShem, give me light.” The deeper request is: “HaShem, make me into a vessel that can receive Your light without distortion, hold it without arrogance, and pour it into the world with purity.”