ISRC stands for International Standard Recording Code. It denotes a certain release of a song. In order to properly register, map, and collect royalties for a song, it is important to include an ISRC with your registration.
On your Songtrust account, you can use our Spotify search tool to grab your ISRCs. Just go to 'menu', 'songs', and 'add a recording.'
If you want to double check that you have claimed all of your ISRCs, check this resource: https://isrcsearch.ifpi.org/#!/search