For a long time, my name didn't have an "accept rate" comment below it. Now it does. I've seen that below other people's names, but never understood what that meant. And why did my name not have it, and now it does?
It's the percentage of questions for which you've accepted an answer. Ask 4 questions, accept 3, your rate us 75%.
Part of the reputation / gaming structure on stack overflow is based on if your answer to a question is marked as the "right" answer. On bigger SE sites like StackOverflow.com, some people won't bother answering a user's question if that user doesn't have a decent accept rate. It doesn't feel worth it if the person isn't going to value your answer, or something like that.
You have to have asked 4 questions or more for it to show up, and there is some delay in it's appearance. More on the rules here: How does accept rate work?