A phrase meaning 'as soon as (something/someone) arrives'; used to indicate that an action will be taken immediately upon arrival.
品物が到着次第、発送いたします。
We will ship it as soon as the item arrives.
到着次第ご連絡ください。
Please contact me as soon as you arrive.
部品の到着次第、修理を再開します。
We will resume repairs as soon as the parts arrive.