What is the meaning of Walk-on?

A student athlete who wants to try out for a college sports or athletic team without the benefit of a scholarship or having been recruited.

An actor of a small (or "bit") part in a theatrical production or film, often without speaking lines.

Such a part in a play or film.

One who arrives and boards transport (such as a train or ferry) without having booked in advance.

Source: wiktionary.org