Cool puzzle.
Way Too Many Knights
that makes no sense, how would you get 4 knights unless promoted, but why a knight? and why would black also promote to knight??
It doesn't matter, it's a puzzle, not a real position.
I don't understand. How is this mate in 2?
White has a unique first move so that, no matter how Black replies, White can always deliver checkmate on his second move. The solution only gives White's winning first move. Imagine some of Black's responses and see if White can always mate. 

White to move and mate in two