Tell me about is a feature in Fallout, and something that was originally planned for Fallout 2 but cut before the game's release. It is an option for NPC dialogue, where if you click the Tell me about button and type in a word, the NPC will give a response if he/she has anything to say on that word you entered.
Behind the scenesEdit
If you asked Hakunin about the Elder in Fallout 2, his response was "The Elder is your mother and our leader." This obviously wasn't in the final version of the game, but can be found by examining the game's files.