Queens Gambit.

The queen's gambit gives away a wing pawn for a center pawn. The plan is for black to take and you get a huge center. But black usual declines the gambit to maintain center control. Black can def end with the C pawn.

This is the slav.

This is the queen's gambit declined.

If you are black and you accept the gambit you want to hang on to the pawn because it is your only compensation. 

But many of the options do not work. As you can see the pawn structure is horrible so you should not accept the gambit. 

Here is a perfect

It has the same principle as the sicilian. In the sicilian your wing pawn trades off for a center pawn. Same here. Have a nice day and stay safe.
world.