。
At which counter can I make a reservation?
Literal
Reservation [topic-は] which window [at-で] can-do-[polite-ますか]?
で here marks the location where an action takes place — 'at the window.' できます is the potential form of する, so 予約はできますか literally asks 'is reserving doable?' The topic は fronts 予約 and sets up the question about the location.