With regard to a bolt that secures a door in place and that has a thick knob [gelustera] at its end, a useful implement for a variety of purposes, the tannaim disagree whether the bolt has the status of a vessel, and one may therefore close the door with it, or whether it is considered a cross beam, which would mean that doing so is classified as building. Rabbi Eliezer prohibits its use, and Rabbi Yose permits it.
The case discussed is a neger – a peg or bolt – that has on its end a gelustera – a thick, rounded knob that makes it into a keli – a utensil – that can possibly be used for a number of different purposes (e.g. as a pestle). Rabbi Eliezer says that it is still considered a simple board and we are concerned with boneh; Rabbi Yose rules that, as a keli, it is independent of the house and clearly being used as a lock, which is permitted.
Rashi understands the case of the neger in the Mishna to be one in which this bolt is placed such that the door is connected to the floor beneath the door. According to the Rambam, this neger is placed into rings that are on two side-by-side doors, and the doors are thereby secured. The Bartenura combines both of these explanations.
As explained above, according to Rashi, the gelustera is the ball at the end of the neger that makes it useful for other purposes, and therefore considered a keli. According to the Rambam, the gelustera is a square or round addition to the bolt which is put there to indicate that the neger is to be used as a lock on a regular basis and that it can no longer be used for building. Some explain that the gelustera described is on the thin end of the neger and that it is, in effect, what we would call a key.