What's the Catch? is an achievement in Plants vs. Zombies: Garden Warfare 2. To get it, the player must give five of their stars to the Mysterious Fish, who will then train the player and give them XP.
The achievement name is a pun on the phrase "What's the catch?" used when a deal is suspiciously sided against the proposer to make sure it's not a trick, and catch as in what a fisherman has caught.
This is a very easy achievement to get. All you have to do is save up five stars by doing quests from the quest board. After you get the five stars, go down to the sewers and talk to the fish. After you are done with that, then you should trade your stars to the fish in order to get the achievement.