The Slav Defense is one of the most popular openings with GM’s. This is for two reasons. The first is that it is one of the most solid lines to play against the Queens Gambit and with the Queens Gambit being a regular opening at high level play, many top players have become fans of this opening.
The Slav also allows for lots of different variations so those players that like to be creative and don’t like to play the same variation every game will really enjoy the Slav Defense because it offers just that.
In the second move black looks to defend his pawn on d5 with his c6 pawn. This is done so that his pawn on the e file can stay as needed and not block the way of the light square bishop.