As the others said, it means you have courage, but balls are testicles, and I think the courage or aggression could come from the amount of testosterone produced by them, which does create aggression and forcefulness.
It is not gender specific as women can also, "have balls".
But like people say, it's a vulgarity, and it's slang, Spanish people say cojones, Americans say nuts or balls, some people say stones, we British say, "bollocks" (he's got some bollocks, cast iron bollocks, balls of steel).
= fearless, can't be hurt (unless you kick them in the balls hehe).
But like I say, women can have it said about them too.
Never use it in formal or polite conversation.