“Wow, good catch.”

You're checking a PowerPoint presentation that your boss is going to give tomorrow to the entire company. You notice a mistake on one of the pages and you point it out. Your boss is surprised that there was a mistake and happy that you told him about it. He tells you:

Wow, good catch.

Wow

"Wow" shows that you're surprised. In the example above, you're surprised that there was a mistake.

good catch

When you "catch" a mistake, it means that you notice the mistake before it becomes a problem, or before a lot of other people see it.

Emily caught a mistake in our calculations.

"Good catch" is a phrase that you say when someone catches a mistake. You say this to praise and thank them for noticing the mistake.