Semantic tags are the machine-readable layer of an Apple Wallet pass. The fields a member sees on the pass are just text: "Gate B12" or "Seat 24A" mean something to a person but nothing to the phone. Semantic tags repeat that information in a structured vocabulary Apple defines, event start dates, seat details, airport codes, balances, so the system understands what the pass is about, not just what it displays.
Technically they live in pass.json as a semantics dictionary, attached to the pass as a whole or to individual fields. Apple publishes the full list of keys, and each pass style has tags that make sense for it: transit passes carry departure and arrival details, event tickets carry venue, performer and seat information, store cards can carry a balance.
What the tags buy is system intelligence. Siri can suggest leaving for an event on time, the wallet can order and surface passes more sensibly, and on newer iOS versions the richer pass experiences, such as the event guide on poster layout tickets, are built from semantic data. Apple has steadily moved in this direction: features increasingly key off semantics rather than display text, so a well-tagged pass ages better than one that only looks right.
Google Wallet has no direct equivalent; its pass types carry structured fields by design. For Apple passes, the practical rule is simple: anything the pass displays that has a defined semantic key should also be tagged. It costs nothing at issuance and decides whether the pass participates in what the system does next.