What's the catch?

What does “What's the catch” mean?

What's the hidden problem? Said when an offer seems too good to be true.

• Low key./ˌloʊ ˈkiː/
• You are impossible./juː ɑːr ɪmˈpɑːsəbl/
• Are you mocking me?/ɑːr juː ˈmɑːkɪŋ miː/
• You're having an affair./jʊr ˈhævɪŋ ən əˈfer/
• Not another word./nɑːt əˈnʌðər wɜːrd/
• I owe you one./aɪ oʊ juː wʌn/
• It'll pass./ɪtl pæs/
• I wasn't born yesterday./aɪ ˈwʌznt bɔːrn ˈjestərdeɪ/