Slav Defense: Two Knights Attack is a chess opening with the ECO code D15, part of the Slav Defense family. It goes 1. d4 d5 2. c4 c6 3. Nf3 Nf6 4. Nc3 dxc4. White opens with 1. d4 (the queen's pawn) and Black answers 1... d5. After 8 moves it is White to move. After the last move Black is a pawn ahead in material. 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 5. a4.
After 1.d4 d5 2.c4, Black defends d5 with 2...c6. Unlike the QGD, the light-squared bishop can come out to f5 or g4.
chessGM has a full course for this line: open the Slav Defense: Two Knights Attack course lesson.