There are three main categories of positions. If there are few pawns on the board the position is open. Bishops are often very strong in these positions because they are not blocked. If there are many pawns blocking each other on the board, the position is considered closed. Bishops are restricted by the pawns in these positions, so knights are the dominant minor piece. Many positions have some features of open and closed positions, and are considered semi-open.
In this closed position, Black has the option to completely close the position with ...a3 or open up the a-file with ...axb3. Which move should Black play?
Black opens up the a-file. Now it's time to find a target to attack!
White's passive pieces are helpless as Black will win the b3 pawn and gain a winning advantage.