"What can I say" is something you say when you don't have any other good response to what someone says. Some situations where you can give this answer include:

• When someone says something really good about you, and you agree but you don't want to brag.
• When someone is really angry about something, but you aren't able to fix their problem.
• When someone criticizes you, and you agree that it's true.

You can use it like this:

A: You're such a good singer!

B: Yeah, well, what can I say? It's a gift.