Royalty free means you just need to pay for the item once per end product thus eliminating backend royalties and instead charging a clear and simple one-time fee. This payment covers the usage of that music track for the life of that project.
The music licensed from AudioJungle is intended to be integrated into a larger project such as a part of a youtube video or commercial.
You may not use music in stand-alone music tracks. The tracks are intended to be used as a part of a larger project. For Example: You can’t add lyrics to an audio track and sell it on iTunes.
It is against the license terms to use the track as a part of another musical work. For example you can’t sample parts of the track or add instruments or do a remix of the track for resale. You are also not allowed to add lyrics or sing a vocal on the track and sell it. The music cannot be repackaged and sold as another stock item.