What does "what's the catch" mean?

The definition of catch in such a context as this is:

a hidden problem or disadvantage in an apparently ideal situation

according to the New Oxford American Dictionary.

Examples of common usage:

• OK, I've seen all the benefits, but what's the catch?
• This sounds too good to be true. There's a catch, isn't there?
• Aha, triple interest rates! So, there's the catch!
• There's always a catch. Nothing's truly free in this world.
• Don't fall for their low rental rates. The catch is in their mileage policy.

The example you provided is correctly written:

What's the catch?

The person who asks this questions seeks to know if there is an unseen or understated caveat or unfavorable term or policy in a potential bargain.

If you mean "What's the catch?" then that means you think something is too good to be true, and you're asking, "In this perfect-seeming picture, what is really wrong that I don't see?"

It reminds me of a great cartoon in The New Yorker some years ago. Two birds are perched on a sign in front of a forest. The sign reads: "Bird Sanctuary". One of the birds is casting a wary eye at the sign and saying, "What's the catch?"

The phrase I think you're looking for is "What's the catch?" and it's generally asked by someone who's being presented with a bunch of marketing speak. The phrase comes from the noun form of 'catch', meaning a hook, post, lever or other device designed to grab something as it's moving past and stop it, or to hook into something that's moveable and prevent it from moving.

For example, if someone tells you "Hurry up now and buy this great offer! Phone, internet, cable, and daily in-home massage all for $19.95 a month!", you would be thinking "there's got to be something about that offer that they're not telling me or they'd go out of business from losing money so fast." And that's when you'd ask, "What's the catch?" That is, what else is there about this offer that you haven't told me yet that's going to make me think it's not such a great deal after all? (And that's when you find out that $19.95 is the rate for the first month, after that it's $900 a month with a mandatory 2-year contract.)