noun
order of arrival; first-come-first-served basis
Used for systems where priority is given according to the order people arrive or apply. Common in event registration, limited-quantity sales, and seat reservations.
チケットは先着順で販売します。
Tickets will be sold on a first-come-first-served basis.
座席は先着順ですので、お早めにお越し
Seating is on a first-come-first-served basis, so please arrive early.
Compound of 先着 (せんちゃく, 'arriving first') and 順 (じゅん, 'order'). The meaning is transparent from its components.