A "hash tag" is slang for the "pound" symbol, A.K.A. "#," A.K.A. a very very small tic tac toe board found on a keyboard and your phone. Used on twitter more than anywhere else now.
Tweets are frequently preceded by a hash tag. Grandpa calls a hash tag a pound sign. Young douches who use twitter call a pound sign a hash tag.
The battlecry to hipsters.
Larry: "My outfit for the dance is amazing, hash tag swag."
Rob: " Shut up hipster.