Dutch Defense is a chess opening with the ECO code A80. It goes 1. d4 f5. White opens with 1. d4 (the queen's pawn) and Black answers 1... f5. After 2 moves it is White to move. The lesson on the board explains every move of the line and lets you play it yourself; it is free.
In chessGM's opening book this position continues with 2. g3, 2. c4, 2. Nf3, 2. Bg5, 2. Bf4.
Black answers 1.d4 with 1...f5, grabbing the e4 square. It is ambitious and aims for a kingside attack, but it weakens Black's king a little.
chessGM has a full course for this line: open the Dutch Defense course lesson.