reserved flight; reserved ride
Refers to a flight or ride that has been booked in advance. Commonly used for airlines, buses, and other scheduled transport.
I arrived at the airport early so I wouldn't miss my reserved flight.
This bus is a reserved ride, so please book your seat in advance.
便 is a suffix meaning 'flight' or 'service' (e.g., 航空便, バス便). 予約便 combines it with 予約 to specify a reserved service.
Compound of 予約 (reservation) and 便 (flight/service). The exact historical derivation is straightforward; the word is a modern compound used in travel contexts.