Modern Defense is a chess opening with the ECO code B06. It goes 1. e4 g6 2. d4 Bg7. White opens with 1. e4 (the king's pawn) and Black answers 1... g6. After 4 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 3. Nc3.
Black plays ...g6 and ...Bg7 early, letting White take the centre and attacking it later. It is flexible and hypermodern.
chessGM has a full course for this line: open the Modern Defense course lesson.