"Royalty-free music," also called "Stock music" or "Library music," represents music that you don't have to pay any licensing fees (i.e., royalties) for each time you use it. Instead, you can buy a single license to use it as many times as you want, for all audiences or copies of your project, according to the rights granted by the license you purchased.
For complete information, I invite you to visit the licensing page.
