Alekhine Defense is a chess opening with the ECO code B02. It goes 1. e4 Nf6. White opens with 1. e4 (the king's pawn) and Black answers 1... Nf6. 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. e5.
Black answers 1.e4 with 1...Nf6, inviting White's pawns to chase the knight. The idea is to make White over-extend, then attack the big centre.