What does # mean on a guitar tuner?
# means a sharp note.
More specifically the string you're tuning is sharp (higher pitched) that it should be. This is because there is too much tension on the string (the string us too tight) try loosening it by twisting the string's tuning peg (the twisty screw thingies at the far end of the fretboard) slightly clockwise.
Typical standard tuning is:
E (top string, lowest sounding note)
E (bottom string, highest sounding note)