What you say when someone has done something extremely well. Comes from actual darts playing, if a player hits three triple twenties or somthing, other players or spectatores say 'tidy darts' or 'good darts'. Also comes from the Welsh slang 'tidy', meaning 'nice one', 'good' or 'that's great'.
Katie: I won the a competition today.
Andy: Tidy darts.