When you use someone else’s music in your video, they basically have full rights to decide what happens, since it’s their original content and not yours. YouTube has an algorithm that automatically finds this music without any human input, so sometimes it’s erroneous (my friend has a gaming channel and he’s always baffled that it found a rap song copyright in the star fox credits). But basically, when they find a song in your video, they can either
A) Leave it be and put ads on your video, all the revenue of which will go to them and not you or,
B) They can put in a formal copyright claim and have your video removed.
If A happens, it’s called a claim, and you’re fine. The worst that will happen is a small ad might appear at the beginning of the video. If B happens, it’s called a copyright strike, and if you accumulate 3 of those within 180 days (I believe, it may be 90), then your channel will be banned and you can’t create a new one. A claim is NOT a strike. You can have infinite claims on your channel with no negative effect. But if you get a Strike, you wanna watch out. If you get a copyright notice and you think it’s erroneous, you can dispute it with youtube or the creator.
Hope this helps!