A prop is something you use as a support, like a broom that serves as a prop to hold a door open. A prop is also something that goes on stage, like a garbage can in a Beckett play.

/prɑːp/ -pp- to support something physically, often by leaning it against something else or putting something under it: I propped my bike (up) against the wall. She was sitting at the desk with her chin propped on her hands.

Cambridge Free English Dictionary and Thesaurus.

In the context of theater and film, the word prop actually comes from a shortening of the word property, which can be used to mean the same thing (though this is now rare due to how common the use of prop is).