Pirc Defense is a chess opening with the ECO code B07. It goes 1. e4 d6 2. d4 Nf6 3. Nc3 g6. White opens with 1. e4 (the king's pawn) and Black answers 1... d6. After 6 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 4. Be3, 4. Nf3, 4. f4.
Black answers 1.e4 with 1...d6, then ...Nf6, ...g6 and ...Bg7. Black lets White build a centre and counterattacks it later.
chessGM has a full course for this line: open the Pirc Defense course lesson.