Dutch Defense is a chess opening with the ECO code A84. It goes 1. d4 f5 2. c4. White opens with 1. d4 (the queen's pawn) and Black answers 1... f5. After 3 moves it is Black 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... Nf6.
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.