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.
An issuer account is created in the Google Pay & Wallet Console: sign in with the Google Account that will hold the admin role, add the business name, and start the Google Wallet API. Every new account begins in demo mode, where only users with the admin or developer role, or listed test accounts, can save its passes, until Google grants publishing access through the dashboard's "Get publishing access" steps.
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.