"No kidding" means "That is true" or "I agree":

A: There's something seriously wrong with her.

B: No kidding.

You can also ask "No kidding?" which means "Really?":

A: I'm moving to Amsterdam.

B: Really! No kidding?

A: Yeah, it's true. I'm moving at the end of September.

"No kidding" is a casual phrase that you can use with friends, family, coworkers, or even people who you've just met.