In the Classical Variation...
Black wants to 1. get a solid pawn-structure
2. place his bishop here
To control the long diagonal
3. Castle Kingside to protect his King
White wants to 1. Castle Kingside to protect his King
2. Prepare an attack on the Black’s Kingside
3. Change dark-squared Bishops
And prepare an attack on the Black’s Kingside too
4. White should move a Knight to b3
If he takes this Knight
It’s bad for his Bishop
Because White has good pawn-structure
And it’s hard to put pressure on the f7-pawn
5. White can also move this Bishop here
And put pressure on the e7-pawn