A QR access pass is a QR code issued for a specific scheduled visit — bound to a time window, a visitor, and optionally a location — that can be scanned at a reader or shown to a human door-person.
The QR itself usually encodes either a signed token (short, verifiable offline) or a URL to a verification endpoint (longer, requires connectivity).
Time-bound QR passes expire automatically, which is why they're preferred over pre-shared codes for one-off visits.