In official music biz terms, the MLC is a U.S.-based Mechanical Rights Organization (MRO) designated by the U.S. Copyright Office to issue and administer the digital audio mechanical blanket license in accordance with the Music Modernization Act to DSPs (digital service providers like Spotify and Apple Music) in the United States.
What this means for you, the independent artist, is that the MLC was created to help songwriters in the U.S. collect mechanical royalties generated from streaming. Since these royalties can be difficult to collect on your own, the MLC is an extremely helpful organization, especially for indie artists.
Every time a song you’ve written is manufactured on a CD, downloaded on a digital music retail site, or streamed through services like Spotify and Apple Music, you’re owed a royalty from that transaction. This is called a mechanical royalty.