Strategy & opening play
Development
Development brings pieces from their starting squares to useful positions and prepares their cooperation. In the opening this usually means activating knights and bishops, securing the king and connecting the rooks. Counting moved pieces alone does not tell you how well they stand.
A practical example
After e4 and Nf3, Bc4 can activate a bishop and prepare kingside castling. A queen that repeatedly hunts pawns may lose development tempi when the opponent attacks it with newly developed pieces.
A common mistake
Applying opening rules mechanically. An immediate threat or winning tactic takes priority. Even a developed knight may be poorly placed if it controls no useful squares.
Try this in your next review
After the opening, ask which piece is not participating. Can you activate it with a threat? Compare your piece activity and king safety before starting an attack.