A is a payment for the use of something you own, a right, not a thing.
Before any of the detail makes sense, fix the core idea. A royalty is money paid to the owner of a right every time that right is used. You are not selling the music away; you are letting someone use it and being paid each time they do.
The thing being owned is a right, the legal permission to copy, perform, broadcast, or pair the music with something else. Whoever holds that right gets paid when it is exercised. That is the whole engine of the music business: rights generate royalties, and royalties flow to whoever owns the rights.
This matters because almost every figure you will ever advise on is, underneath, a royalty of one kind or another. Learn to see them and the statements stop being a wall of acronyms.