"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.