This term refers to films that don't belong to anyone and are not in copyright, which means that you can watch and play them without paying. Officially there is no list of public domain films. This is because it is very difficult to be sure that a film doesn't breach copyright or other guidelines; for example, the film itself may not belong to anyone, but it could contain lines from a book or melodies from a song that is still protected by copyright. So it is very hard to get anyone to confirm that the film you're interested in is, in fact, in the public domain.

There are various lists such as the Public Domain Movie Database, which are useful in giving you an idea of what's out there, and if you type the key words 'public domain film' in a search engine you can find others. A reputable list will contain words like 'this is a list of films that are thought to be in the public domain' - ie, they don't pretend to be official.