Mainly music that is free to listen to, download and share. But other restrictions may apply.
Although the below links are to music that may not be subject to copyright, other restrictions may apply, such as if you use it in somthing else like a video, that you have to give attribution (acknowledge who the owners/creators are)
Some web sites have Royalty free music. That can mean you pay for it once and can use, play, whatever it, multiple times. Or a certain number of times in certain situations. You always need to check.