“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.
Wow, good catch.
"Wow" shows that you're surprised.
People often say "wow!" in a a really excited tone of voice, but you can also say it in a relaxed and calm voice if you're reacting to something that's only a little surprising.
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.