An ISWC (International Standard Musical Work Code) is a unique, universal, and permanent identifier for a specific composition or work (not the sound recording). It is used by Performance Rights Organizations and Publishers to track songwriter royalties throughout the music ecosystem.
Learn more at https://www.iswc.org/what-iswc
ISWCs are assigned by Registration Agencies (Music Societies) under the administration of CISAC (International Confederation of Societies of Authors and Composers) and are correlated with a Songwriter's IPI Name Number. The ISWC identifies the specific composition associated with a specific songwriter's IPI.
Providing ISWCs are optional when submitting your songs for distribution (since not every song or composition actually has one). However, you should make every effort to try and find out a song's ISWC, especially if you are not the songwriter, as ISWCs are one of the ways that royalties end up being paid to songwriters.
You can search for a given song's ISWC at CISAC here: