Wendy is correct. 'kidding' means something like 'joking' or 'teasing'. It's when your friends tell you something that isn't true to play with you. 'kidding' is light-hearted, not deliberate lying or cheating. So, if they are 'not kidding', it means they are telling the truth. So, you would usually say that to refer to something that is a little unusual, or possibly untrue.
Another similar thing to say is 'Are you kidding?' - which is when you want to make sure that someone isn't teasing you.
In your question - you should write 'mean', not 'means'. i.e: What does xxx mean?