These are the payments that the songwriters get when their songs are downloaded, sold on, and reproduced for digital formats. They can be computer games, musical toys, music videos, ringtones, etc.
In other words, any copyrighted audio composition out there that is created without human performers fall into mechanical royalties. The statutory rate or the standard mechanical rate for a sale is decided by the US Copyright Office.
Artists earn money through this by giving the music mechanical license to people for reproducing and distributing their music