Let's look at two uses of catch:
As a verb catch can mean to capture; not allow a person, animal or thing to escape:
"The fisherman caught a fish in his net."
"The police are still trying to catch the man who escaped from jail last night."
What's the catch is not an idiom about fishing!
We use it when something sounds good, but we are worried about any hidden problems. We are asking what are the drawbacks. When something is too good to be true we ask what's the catch?
"OK, I've seen all the benefits, but what's the catch?"
"The restaurant is offering free lunch, but what's the catch?"
