“Yeah, no problem.”

You're sitting in a coffee shop. Someone who you don't know asks if you will watch over her stuff for her. You don't mind, so you say:

Yeah, no problem.

Yeah

This is a more casual way of saying "yes". Use it in spoken conversation when you don't need to sound too serious.

No problem.

"No problem" means "yes" or "sure". It is a response that you say to someone who asks you to do something:

A: Could you hold that for me?

B: Sure, no problem.

You can also say "No problem" when someone thanks you for doing something:

A: Thank you so much!

B: No problem.