It means that you want to know what the person thinks of a suggestion or proposal made to them. Alternatively, if someone has questioned you on something you can ask that to mean you want to know why they are curious.

In addition to Ryan's meanings, it can also occasionally be used a slightly rude way to question somebody's interest in something. e.g. someone says 'I see you have a limp', and you reply 'so, what about it?'. That suggests that you are not very happy about them saying that, and are wondering why they asked.

Agree with the other two responses. There are a few possible inferences. 1. You are asking what the other person thinks of your proposal/idea/request in a genuine way to prompt an answer. It can also be used in advertising where an ad might say: "So what about it?" to encourage people to engage in something or buy their product as it's an open question or, 2. It's more of a rhetorical question and you're wondering why the other person just said what they said. It sounds a bit defensive.