Final Fantasy XIV's Worst Mechanic (isn't real and can't hurt you)

Before Purse of Riches and its insane downtime, whenever people discussed the craziest parts of fishing in FFXIV, Fish Jail always came up. But what is Fish Jail, and do you actually need to do it? The answer to the second question is "no." Lots of screenshots taken from the Carbuncle Plushy fish tracker: https://ff14fish.carbuncleplushy.com/