Starting position
The moves 1.e4 e5 2.Nf3 Nc6 3.Bb5 Bc5 — the Classical Defense: Black's bishop aims at f2. White plays the pawn to 4.c3 and prepares the d4 break in the center.
The opponent's mistake
4...d6? looks solid: the pawn supports e5. But the pawn has left d7, so the knight on c6 is now pinned to the king, and the pawn move 5.d4 attacks the bishop on c5.
How the trap works
After 5...exd4 6.cxd4 Bb4+ 7.Kf1 Bd7 8.Qa4 the queen hits the bishop on b4, which only the knight on c6 defends — and the bishop on b5 attacks that knight. The piece can't be saved: 8...a5 9.a3 Nxd4 10.Nxd4. If the bishop retreats to b6 — 6...Bb6 7.d5: the pawn attacks the pinned knight on c6, and a piece is lost again.
1.e4 e5 2.Nf3 Nc6 3.Bb5 Bc5 4.c3 d6 5.d4 exd4 6.cxd4 Bb4+ 7.Kf1 Bd7 8.Qa4 a5 9.a3 Nxd4 10.Nxd4
Result
White is a piece up — a decisive advantage.
How to defend (as Black)
Play 4...Nf6: the knight attacks the pawn on e4, and White has to defend it.