Just like a story has a beginning, a middle, and an end, a chess game is divided into three distinct phases. Knowing which phase you are in tells you exactly what you should be doing.
The first 10-15 moves of the game. The board is full, and both armies are waking up.
Starts when the pieces are developed and the Kings are castled. This is where the heavy fighting happens.
Starts when most pieces (especially Queens) have been traded off. The board looks empty, but every move is critical.
Beginners often mix up the goals. They try to attack the King in the Opening (before they are ready), or they hide their King in the Endgame (when it should be fighting).
Remember: Opening = Safety. Endgame = Activity.