Public Domain means the music/book/movie/character isn't covered by copyright anymore, either because protection ran out or wasn't renewed. When that happens, everyone are free to use, modify, distribute, sell as they wish.
For instance: You are free to write and sell stories about Dracula, Sherlock Holmes, Tarzan, Robin Hood, (early) Mickey Mouse, Alice (In Wonderland), Cthulhu, Scrooge, Rosie the Riveter, Aladdin, King Kong and many more. This year Tintin and Popeye was added to the list. (Before you start writing, make sure to be inspired by the unprotected works only, as later versions of the character might still be protected.)
Other notable additions: music like "Singin' In The Rain", "Bolero" and "Happy Days Are Here Again", books by William Faulkner, Hemingway, Agatha Christie and Steinbeck.
Here is an article with more info on this year's additions: https://web.law.duke.edu/cspd/publicdomainday/2025/