An ISWC code is short for The International Standard Musical Work Code (ISWC) which is a unique identifier for musical works. It is an 11 digit code for cataloging individual work compositions (or songs) rather than recordings.
ISWC's are typically assigned by a collection society (such as ASCAP, BMI, SOCAN, etc.) to a musical work. If you produce your own music and release it to the public in any form you'll need an ISWC, it links your work back to your publisher.
Having an ISWC separates your composition from similar works and lets people know who needs to be paid when that composition is used commercially.
A composition only news a single ISWC even if it's an adaptation of an original.