A Team ID is the ten-character alphanumeric identifier Apple assigns to an Apple Developer account. Every asset in that account, from app IDs to pass type identifiers and signing certificates, belongs to that team.
In Apple Wallet, the Team ID appears in every pass. The pass.json file declares it in the teamIdentifier key, and Wallet checks that it matches the team behind the certificate that signed the PKPass. A pass signed with a certificate from one team but declaring another team's identifier is rejected before it ever reaches a wallet.
The Team ID is also where the ownership question becomes concrete. When a brand registers its pass type identifier in its own Apple Developer account, the Team ID in every issued pass is the brand's, not the wallet vendor's. That is what makes a future pass migration clean: the identifiers stay put, only the platform behind them changes.
You find your Team ID in the Apple Developer portal under membership details. Enrolling as an organization requires a D-U-N-S number and legal-entity verification, which is worth starting early since it can take days before the account, and with it the Team ID, exists at all.