Keilim, Chapter 11, Mishnah 2. This chapter is devoted to the laws of metal utensils, and our mishnah adds another rule to that discussion: which metal objects are considered independent kelim, and which are not.
The opening clause reads: "kol klei matachot", all metal implements, "sheyesh lo shem bifnei atzmo", that bear a name of their own, "tamei", are able to contract tumah. The meaning is this: whenever an item is identified as an entity unto itself and not as one piece of some larger utensil, it holds the status of a keli and takes on tumah by itself.
Consider a knife. Nobody thinks of a knife as a fragment of something bigger; it does its work unattached to anything. Since it functions as a full utensil while standing alone, tumah can come upon it independently.
Compare that with an item whose very name announces that it belongs to something else, such as the arm of a menorah. A piece like that is not treated as a utensil unto itself, and as long as it lies separate no tumah can touch it. Only after it has been fitted into the keli for which it was made does it enter the realm of tumah.
Two clarifications belong here. The first: when the mishnah speaks of an implement bearing a name of its own, it refers to flat implements, those with no cavity able to hold anything. Where a utensil does have a receptacle, neither its name nor its function matters at all; it takes tumah in any event, since the presence of a hollow on its own already renders the object a self-standing keli.
Second, the mishnah is speaking specifically about metal, since metal is the material whose utensils can become tamei even when they are flat and even when they lack any container at all.
The mishnah now enumerates objects that stand outside this rule altogether. Each of them has a name and a use of its own, yet none has any connection to tumah and none can ever become tamei: "chutz min hadelet", apart from a door; "umin haneged", and the stopper that holds a door in place; "umin haman'ul", and a lock; "vehapotah shetachat hatzir", and the hollow socket lying beneath the door's pivot; "vehatzir", and the pivot itself; "vehakorah", and a ceiling beam of a house; "vehatzinor", and a water pipe.
Observe how many of these are pieces belonging to a door: its hinge, its pin, the hollow in which that pin rotates, the fitting from which it hangs. The mishnah supplies its own explanation for their exclusion: "shena'asu lakarka", their manufacture was for the sake of being fixed into the earth, or into a building which is itself fixed there. Anything produced to be set permanently into the earth lies beyond the reach of tumah.
So the principle that emerges is clear. Metal utensils that have a name of their own are susceptible to tumah, but once an object is manufactured to be part of the ground or of a structure joined to the ground, something that is mechubar lakarka, connected to the earth, it falls out of the category of a keli. Every item listed in this mishnah, though it is metal and though it has a name, can never become tamei.