ISWC stands for International Standard Musical Work Code. This 11-character alphanumeric code is used to uniquely identify a musical work.
Songs receive ISWC's when they are registered at a performing rights organization (PRO). If your song is brand new, Songtrust helps register it to be assigned an ISWC.
Each song should only have one ISWC. However, arrangements, adaptations and translations should receive their own unique ISWC's. ISWC's are issued by your PRO, so if your works are already registered with your PRO, they have been assigned ISWCs. An ISWC looks like this: T-123.456.789-Z and you can find them by searching for your songs in your PRO's respective repertory on their website.