Chess Opening Theory/1. e4/1...e5/2. f4/2...d5/3. exd5/3...e4

= Falkbeer Countergambit =

3...e4
After giving the pawn on d5, the e4 square is now free so Black pushes the e pawn instead of taking on f4. The idea is to ruin White's plan of opening the f file. The e4 pawn will also be quite a nuisance for White as it prevents natural development of the knight on f3. White will thus try to get rid of this pawn as soon as possible.

Theory table
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1. e4 e5 2.f4 d5 3.exd5 e4