1. tell me about it, talk to me, discuss, give me an update, tell me how you feel
sit down and tell me (all) about it
If you ask someone to 'tell you all about' something, it can be an offer to listen to their problems and allow them to express how they are feeling or to give you some information about something. This is a casual expression used in social and professional contexts.
2. (sarcastic) I already know, you don't have to tell me, I can identify
"Yeah, I know. Tell me about it!"
The expression 'tell me all about it' is sometimes used to say that you understand what someone is talking about because you have experienced it yourself.