What's the catch asks what hidden problem comes with a good offer. It sounds free, so what's the catch.
It assumes there is one, which is usually a safe assumption. The catch is the hidden condition, and the phrase is asked before agreeing rather than after.
1phrasewhat's the catchAsking what hidden problem comes with a good offer.
“Half price for a year, so what's the catch.”
what's the catch is a question.
Practised in Small Talk & Everyday Phrases