An ISRC code is a unique code assigned to each song you upload.
DistroKid has been appointed by the US ISRC Agency to assign ISRCs, and will automatically generate new DistroKid ISRC codes for every song you upload. It's free and automatic.
If you'd prefer to specify your own ISRC code(s), you can do that if you've got a "Musician Plus" or "Ultimate" account.
More info on how to use your own ISRC code(s) with DistroKid.