Royalty-free describes a licensing model, not an absence of rights. The user pays once, or nothing, for permission to use a track within agreed limits, without owing further payments each time a video is viewed or the track is played. The music remains protected by copyright, and the licence defines where, how and for how long it may be used, for instance online video only, no resale of the track on its own, or a credit in the description.
Libraries such as YouTube’s Audio Library show the range: some tracks come under a standard licence that needs no credit, others under Creative Commons licences that require attribution. Licences that look similar can differ in important details, such as whether commercial use, broadcasting or public performance is covered.
Royalty-free is also not the same as public domain or “copyright-free”. Depending on the country and on how the music is registered, playing it in a shop or café may still involve licences from collecting societies. For a specific project, reading the licence closely and asking a professional is the safest course.