An issuer ID identifies a Google Wallet issuer account, the account that owns everything a program creates on the Google side. It is Google's rough counterpart to Apple's pass type identifier: the namespace all of a program's passes live under, and the anchor for who controls them.
Under an issuer account, Google's model splits a pass in two. A class is the template: layout, branding, program-level content, shared by every pass in the program. An object is one member's pass: their identifier, their balance, their state, always referencing a class. Editing a class changes every pass built on it; editing an object changes one member's. Both IDs are prefixed by the issuer ID, which is what makes ownership concrete.
Members receive passes through a save link, a signed token that tells Google which object to add, and updates flow by writing to the object through Google's API. There is no per-device registration to maintain as on the Apple side; Google delivers changes to wallets itself.
Ownership is the question to ask before signing with any provider. Classes and objects cannot move between issuer accounts, so if a program's passes live under a platform's issuer ID, leaving means reissuing on the Google side, a real cost that pass migration covers. Knowing whose issuer ID a program runs under, before launch, is one of the cheapest pieces of due diligence in wallet loyalty.