Mechanical royalties are a kind of publishing royalty that a songwriter can earn.

Wikipedia says:

The term “mechanical” and “mechanical license” has its origins in the “piano rolls” on which music was recorded in the early part of the 20th Century. Although its concept is now primarily oriented to royalty income from sale of compact discs (CDs), its scope is wider and covers any copyrighted audio composition that is rendered mechanically; that is, without human performers.

In a nutshell: every time a song you’ve written is manufactured to be sold in a CD, downloaded on a digital music retail site, or streamed through services like Spotify and Rdio, you are owed a mechanical royalty.