French Defense is a chess opening with the ECO code C00. It goes 1. e4 e6. White opens with 1. e4 (the king's pawn) and Black answers 1... e6. 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. d4.
Black answers 1.e4 with 1...e6 and then ...d5, attacking White's centre. Black accepts a little less space for a very solid position.
chessGM has a full course for this line: open the French Defense course lesson.