It means to talk about somebody when they are not there. It is almost always used with negative things that you are saying about somebody. Here are some examples: 'If somebody has a problem with me I wish they would come and tell me instead of talking behind my back!' 'It is not nice to speak behind somebody's back' In your phrase 'What have you been saying about me behind my back?' It means what have you been saying about me while I wasn't there. Hope this helps! Let me know if you have any questions!

It's metaphorical - it means that you talk about somebody without them knowing you are doing it; it's usually unfair; talking about them in a negative way. http://dictionary.cambridge.org/dictionary/english/behind-sb-s-back

It simply means that you have been saying something - usually something bad - about a person without his or her knowledge. Example: You are sitting in a café with your friend Fred and you have the following conversation: "You know that new girl in Marketing?" "Who - Alison?" "Aye; that's the one ..." "Well, what about her?" "I don't think she'll last long in that job: she's lazy!" In that example, you have been talking about Alison behind her back, which is very bad form.

(talk about) sbdy behind their back = gossip (chismear) about sbdy

if you stand behind someone and you talk with someone else about the person in front of you.