To "catch" someone means to get a chance to speak with them.

For example:

A: Have you asked Tony about getting you access to the expenses database yet?

B: No, I wasn't able to catch him yesterday.

Oh, I'm glad I caught you. I have a question...

You can use this phrase with friends, coworkers, or other people that you've met before. Don't use this phrase to talk about getting the chance to meet someone for the first time.