Chess Opening Theory/1. d4/1...d5/2. e4/2...dxe4/3. Nc3/3...Bf5

= Blackmar-Diemer Gambit (BDG): Zeller Defense =

3. Bf5
Bf5 defends the pawn on e4 and develops the queenside bishop, with intentions of keeping the pawn as it is a thorn to White who can't play Nf3 or Bd3, otherwise the knight/bishop will be taken by the pawn.

Responses
White usually responds with 3. f3?!, which attacks the e4 pawn albeit blocking the development of the knight to f3, which can be rectified after exd3. A rare option that is preferred by computers is g4, kicking the bishop away with the intention of creating a pawn storm with h4 and h5, forcing Black to play defensively.

Other than that, other options can block pieces from developing, get kicked away by pawns, or just do nothing.

Theory Table

 * 1) d4 d5 2. e4 dxe4 3. Nc3 Bf5