Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bc4/3...Nf6/4. Ng5/4...Nd5/5. d4

= Two Knights Defence =

5. d4
White's best move 5. d4 refrains from capturing the Black knight for one turn, and instead defends their own knight with the dark-squared bishop. Black also finds themselves unable to move away their undefended knight because of its pin to the f-pawn.

The move 6. exd5 will threaten the other Black knight as well so the most Black can do is at least gain one knight in compensation with 5...h6 or sacrifice their knight for a pawn with 5...Nxd4.

Theory table
'''1.e4 e5 2.Nf3 Nc6 3.Bc4 Nf6 4.Ng5 Nd5 5. d4'''