Center Game is a chess opening with the ECO code C20. It goes 1. e4 e5 2. d4. White opens with 1. e4 (the king's pawn) and Black answers 1... e5. 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... exd4.
After 1.e4 e5, White plays 2.d4 at once. After 2...exd4 3.Qxd4, the queen comes out early and can be chased.