The King’s Gambit Declined
Is a variation of the King’s Gambit opening
White starts by pushing the King’s pawn
And Black replies symmetrically
White sacrifices a pawn with f4
And Black does not take this pawn
Which means that he declines the gambit
Instead, he has two options
To play the Bishop here
Or to break in the center immediately
Both of these moves are the King’s Gambit Declined