How does this affect what royalties you earn?
The Songwriter(s) is the person who composed the song, whether it be the lyrics or the melodies.
The Performer(s) is the person or band that performs the song, whether that be recorded or live.
Each are have different rights and separate royalty streams. The performer(s) can earn on the master recording, while the songwriter(s) will always earn on the composition, regardless of who the performer is. You can learn more about the difference here.
For example, the song "Free Fallin'" was written by Tom Petty an Jeffrey Lynne. One of the most famous versions of this song was performed by the artist John Mayer. While John Mayer is entitled to royalties for the master recording, Tom Petty and Jeffrey Lynne earn royalties on his performances for the composition. Not only that, but John Mayer must pay Tom Petty and Jeff Lynne a compulsory mechanical license for each copy sold of his version of their song.
Songtrust does not collect on behalf of the performer unless they are also the songwriter. You can not register cover songs with Songtrust.