Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...f5/4. d3

The move 4.d3 defended e4 but also threatened to snaffle the f5-pawn into the bargain, because now Black won't have the instant rejoinder ...e4.

4...fxe4 is the only move to justify the Black pawn thrust.