A fish stall is a stall, which may be stolen from at level 42 Thieving, granting 42 Thieving experience. Stock replenishes roughly every 10 seconds. Thieving from the stalls in Rellekka requires completion of The Fremennik Trials quest.
Thieving from the Miscellania stalls will lower your approval rating if you are managing this kingdom. Interestingly, stealing from the Etceteria stall doesn't lower your approval rating. Possible loot includes raw salmon, raw tuna or raw lobster.