1) A way of stating that they're talking about something you know very well or agree with.
2) A way of stating that they're actually talking about you. (Often used as sarcasm).
1)
Ben: Punching hurts people Frank: Tell me about it!
2)
Ben: Did you tell her about your "problems"
Frank: Being god-like isn't a problem
Ben: Tell me about it
used as an ironic acknowledgment of one's familiarity with a difficult or unpleasant situation or experience described by someone else.
"My car broke down on the highway in the pouring rain, and I had to wait for hours for a tow truck." "Tell me about it," my friend replied, sighing as they clutched their own dead phone. (Their sigh and clutching the phone imply they're also stuck in a difficult situation).
A common American phrase used to end a conversation due to time constraints. Usually used to be polite.
Friend: ... So after the movie we found the craziest thing in the alley.
Me (about to do something important): Sounds interesting, tell me about it later.
Nickname given to bales of marijuana thrown overboard or out of airplanes. Origin is reportedly from members of the US Coast Guard in the area of Southern Florida.