Starting position
The moves 1.e4 d5 2.exd5 Nf6 3.c4 e6 4.dxe6 Bxe6 — the Icelandic Gambit: Black has given up a pawn for quick piece play, and the bishop on e6 is already active.
The opponent's mistake
5.b3? defends the pawn on c4, but the b2 square is now empty: the long diagonal to the rook on a1 is open. Developing was better.
How the trap works
5...Bc5 — the bishop aims at f2. White covers the rook with 6.Bb2?, but after 6...Ne4 both the knight and the bishop attack f2, and 7.d4 is met by the check 7...Bb4+: White can't block with the bishop on d2, so the king goes 8.Ke2. The queen lines up against it, 8...Qe7, and after 9.f3 Bxc4+ 10.bxc4 the knight jumps 10...Nc3+ with a double check and then takes the queen: 11.Kd2 Nxd1+.
1.e4 d5 2.exd5 Nf6 3.c4 e6 4.dxe6 Bxe6 5.b3 Bc5 6.Bb2 Ne4 7.d4 Bb4+ 8.Ke2 Qe7 9.f3 Bxc4+ 10.bxc4 Nc3+ 11.Kd2 Nxd1+ 12.Kxd1 Qe1+ 13.Kc2 Qxf1
Result
Black has won the queen for a knight and a bishop — a decisive advantage.
How to defend (as White)
The right move was 5.Be2: the bishop develops, and the pawn on b2 still blocks the diagonal to the rook on a1.