All my free music is licensed under a Creative Commons Attribution 4.0 International License. That means that you can use it anyway you want. The only thing you have to do is to give me a credit (for example, put the text: “Music: Sokolovsky Music – File Name. CC BY 4.0. http://www.sokolovskymusic.com/free-music” ) in the description of your video).
AudioJungle is a marketplace of royalty free music and sound effects. At AudioJungle you can find music and sounds for just a few dollars, which includes a Buyer Fee. The site is home to a bustling community of audio composers and producers.
The information about Audiojungle licenses you can find here.
Royalty Free Music is a term that refers to production music that has no additional fees to pay once the music has been licensed. The license fee is paid one time and (depending on the license or company selling it) can be used in a project of yours either a single time or multiple times. Royalty free music licenses and the libraries that provide them are offered on a non-exclusive basis (in other words, they can license the song to you and anyone else on the planet at the same time).