This mishnah continues the topic we have been dealing with: which labors were permitted in the Beis HaMikdash because they are forbidden only by rabbinic decree, and which labors are forbidden by Torah law and therefore were not permitted even in the Mikdash. Our mishnah comes to discuss the hinge - the pin on which the door of a cabinet turns, such as a wall cabinet.
What labor is involved in replacing the hinge?
The commentators disagree about which labor might be involved in setting a hinge into a cabinet. A cabinet is regarded as something that can be moved from place to place and is not attached to the ground, and therefore it is not necessarily subject to the laws of building - for we hold "ein binyan b'keilim," that the labor of building does not apply to vessels. Still, under certain circumstances this might constitute makeh b'patish (striking the final blow). Whatever the labor may be, our mishnah establishes what is considered a Torah prohibition and what is a rabbinic one.
"Machzirin tzir hatachton b'Mikdash":
This cabinet, which holds the door in place, had an upper hinge and a lower hinge. (Although we cannot know whether the hinges resembled those of our own day, the door certainly turned on some sort of hinge.) The lower hinge was not as critical as the upper one, and therefore the mishnah says that one may return the lower hinge to its place in the Beis HaMikdash, since the prohibition against replacing it is only rabbinic.
"Aval lo bamedinah":
Outside the Beis HaMikdash, replacing the lower hinge was forbidden by rabbinic decree. The concern was that even if this does not constitute actual building, and even if it does not constitute makeh b'patish or the making of a vessel, it might lead a person to perform a more significant labor: to take out his screwdriver and tools and drive in nails in order to hold the hinge more securely. The Sages therefore forbade it, and there is no permission to return the lower hinge to its place outside the Beis HaMikdash.
"V'ha'elyon, kan v'kan asur":
The upper hinge may not be replaced, neither in the country nor in the Mikdash, because doing so involves a transgression of derisah (forcing it firmly into place).
The view of Rabbi Yehudah:
Rabbi Yehudah disagrees, and draws a different distinction between the two hinges:
"Ha'elyon - mutar b'Mikdash" - even the upper hinge may be replaced in the Beis HaMikdash, since its prohibition is only rabbinic.
"V'hatachton - mutar af bamedinah" - there is no prohibition at all, even outside the Beis HaMikdash. This is something that is routinely put in and taken out, and the door is not unsteady without it: without the upper hinge the door is unsteady, but the lower one is not so significant, and the door does not fall without it. Therefore it may be replaced even outside the Beis HaMikdash.
In summary: Our mishnah deals with replacing the hinge of a cabinet door - a labor that does not constitute building (for "ein binyan b'keilim"), though some see in it an element of makeh b'patish. According to the Sages, one may replace the lower hinge in the Mikdash alone, and the upper one neither here nor there; and according to Rabbi Yehudah, the upper one is permitted in the Mikdash, and the lower one is permitted even in the country, since it is routinely put in and taken out and the door is not unsteady without it.