expression
when you get to the destination; once you arrive there
A set phrase used to talk about what will happen or what to do upon reaching a place. The たら is the conditional form of 着く.
向こうに着いたら電話してください。
Please call me when you get there.
向こうに着いたらまずホテルにチェックインしよう。
Let's check in at the hotel first once we arrive.