Let's say we have a zombie game (how original - I've heard that even 60% of new games submitted to App Store are Zombie games...!).
Your goal is to survive from all zombies and your objective for this level could be like to get into location A which could be on the second floor of a house.
Now how do you achieve this goal? If you provide only one answer, your game can be quite boring... So provide variation. Variation will also show in your scoring.
Provide variation to how the goal can be achieved and use variation in the objectives. Use first attack and then defend. From killing to protecting or even reviving. You could even flip the goal to create dramatic effect!
Your game needs to have a goal. Even more your player needs to know what it is so keep this information visible.
Objectives are for individual levels or sections of the game. The goal of your game often transforms in objectives. Objectives are also really powerful and maybe also one of the few storytelling methods in a game.
The objective of Tetris to avoid hitting the top of the screen and to create full rows which are then removed.
In a previous page Mechanics you saw what a simple change in the objective - instead of creating rows, you fill the grid with the pieces - can do.
Simple change to basic Tetris would be to create a way bigger area which you would need to fill with the dropping pieces as fast as possible and without leaving any gaps!
What is your twist to the objective?