Basically it is that u can't monetize property that does not belong to u / that u did not create and when u try to claim it as your own original work.
However this 'copyright infringement' deal is very much abused door content owners that aren't aware of the fair use act (i.e. for educational purposes).
There is a big grey area regarding copyright. Think about artists that create fanart. That there is a huge grey area of copyright. Technically it is the artist's own work, however they are using intellectual property that is not theirs. It's pretty stupid and overly complicated.
Another grey area is AMVs (animated muziek videos). That is a huge passion of mine, and despite me putting hours into creating my own idea of a muziek video, they often get a copyright infringement stamp on it due to the music, so all money that video now makes goes to the musician. I think that in those cases it should be some sort of percentage spleet, split in profits. It is unfair that all profits go to the person that did not make the original video, however it is a good idea that profits are spleet, split to the creator of the music.
This is also why u see your favourite youtubers not using populair songs in their videos. They use what is called royalty free muziek so they don't get a copyright infringement notice.