Starting position
The moves 1.e4 e6 2.d4 d5 3.e5 c5 4.c3 Nc6 5.Nf3 Bd7 — the Advance Variation, Euwe Variation. Black presses on the pawn on d4, and the bishop on d7 stands behind the knight on c6.
The opponent's mistake
6.Bb5? looks active: the bishop attacks the knight on c6. But it lands on the same diagonal as the bishop on d7, and only the knight itself blocks it.
How the trap works
6...Nxe5! — the knight takes the pawn on e5, and now the bishops face each other: the queen defends the bishop on d7, while the bishop on b5 is undefended. The knight also threatens to take the knight on f3 with check. White saves the bishop by trading, 7.Bxd7+ Nxd7, and Black is a pawn up.
1.e4 e6 2.d4 d5 3.e5 c5 4.c3 Nc6 5.Nf3 Bd7 6.Bb5 Nxe5 7.Bxd7+ Nxd7 8.O-O Ngf6 9.Bg5 h6 10.Bh4 Be7 11.Nbd2 O-O 12.Re1 cxd4 13.Nxd4 Rc8
Result
Black has won the pawn on e5 — the one that held White's whole pawn chain.
How to defend (as White)
The right move was 6.Be2: the bishop stays off the d7 diagonal, and the knight can't take the pawn on e5 — the knight on f3 defends it.