ISRC stands for International Standard Recording Code.
These are used to track the copyright of a recording and help you make sure you get paid for public playing or streaming of your music.
You need to submit these wherever you upload your music and to your performing rights association where you register your works.
A recording should only ever be issued one ISRC and that should be used wherever the recording is published.
In most cases these days, the ISRC codes are issued by your aggregator. However, you may be planning a CD release or releasing only on sites like Bandcamp, Juno or Beatport. (Let’s face it, they pay better and are less problematic than streaming services – you know, the money doesn’t fund weapons manufacturers). In which case, you’ll need to get it somewhere else
STEAM Mastering is a registered ISRC manager with A.R.I.A and will happily issue an ISRC to any customers who require one at no charge. (you should never be charged for ISRC codes, so be wary of anyone asking for money)
For more information on ISRC codes checkout this page on wikipedia.