There are many different types of licensing for music, but I am going to focus on the two most important ones for songwriters. These two include mechanical and synchronization licensing.
Mechanical licensing is a license that a person gets when they want to record and release a song by another person. In this license they are given the rights to make copies of the song and release the song to the public for a percentage in royalties. The percentage is negotiated by the publisher or it is the US statutory rate. The US statutory rate which is 9.1 cents for a song under 5min and 1.75 cents per minute for songs longer than 5 min. You can get a mechanical license from either Harry Fox Agency or the publisher of the song. Harry Fox Agency is a provider of mechanical licenses and a royalty distributor to publishers. The royalty percentage there is usually the US statutory rate.
Synchronization licenses or sync licenses are for video recording. This is the license needed when using a song for a YouTube cover video or movie. In exchange for a percentage of royalties, you get the rights to make a copy of the song and put the song to video. These are handed out by the song’s publisher only with a royalty that is negotiated.