Open, closed, and semi-open games
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.
White is attacking Black on the queenside, hoping to open up lines against Black's king. Which pawn move helps keep the position closed?
Black gains a tempo against White's bishop and keeps the c-file closed. White is ready to try and open lines with a5. Can you find a pawn move on the queenside that prepares to close the position if White tries advancing a pawn?
Black plays a strong waiting move. White tries to open up the queenside - let's close it down!
Black's position isn't perfect, but by keeping the game closed, Black has increased chances for a successful defense.