To a first approximation: In open games the central pawns have been swapped off so the board is open and the bishops rule. In a closed game the central pawns are locked up and the knights are more important.
A basic explanation would be that an open game transpires after the moves 1e4, e5. and a closed game begins 1d4, d5. But the terms can be used more loosely and do not necessarily apply to the response of black.