Youtube Content ID is Youtube's Rights / Royalties & tech that determines ownership over a piece of content, such as music. Whomever is the owner, can manage who uses that music online (like licensing it under a Youtube video.)
There must always be 1 owner who can sign off on it. All music libraries need to manage it so they can sub-license it to youtubers.
Youtube Content ID additionally decides who is the owner over a piece, by who uploads it first. If you use royalty free melodic, harmonic, or drum loops - it doesn't matter if you have the license to use it. Whomever uploaded it first will lay claim to ownership over it. Thus, whatever you upload from that point (after ownership was made over a loop), it will be taken down as a copyright infringement. More information over sample usage can be found here.
