You might be given a problem of "we want a new social feature to increase retention" (which means they want users to interact with each other in a way that keeps them coming back to the game everyday.)
Your job will be to think up how this feature can work in your game. Maybe you have a farming game, and so your idea for this feature is a state fair style competition where your vegetables get judged against other peoples vegetables, and the winner gets a cool prize.
Now that you have the idea, you need to document it all in a way that it can be made by the programmers:
You will need to figure out what questions need answers, and then answer them:
- How often does this event occur?
- How do you choose which people the person competes against?
- How do you rate the vegetables, what makes one zucchini better than another, and also what makes one zucchini better/worse than one tomato.
- What are the prizes?
- Do the prizes scale with how big your farm is? Or are they a flat prize?
You need to create a document somewhere where all these answers live, and then once it gets reviewed you will probably have to change a bunch of it, and then finally it will go to the software engineers to build the feature, and the UI Designer/Artist to put the UI in front of the users, etc...
When you are just starting out as a game designer though, you will have much smaller tasks. Your job might be to go through the different vegetables and define a scoring system on a per vegetable basis, and make sure that the scoring system is balanced for each of the vegetables. Or you might be asked to come up with the story behind the competition, why is it happening? etc..