The optimal way to use the Fitbit app is alongside one its devices (for sale on its webpage), which help you measure your physical activity throughout the day: the steps you take, stairs you climb, calories you burn, etc.
Besides measuring your physical activity, Fitbit also helps you keep a record of the water you drink and the food you eat. The app recommends a daily amount of water you should consume, so each time you have a glass you should note it down on the app. Fitbit also recommends a certain number of calories you shouldn't go over in a day, and lets you write down the food you eat.
Fitbit, in short, can become a central switchboard for keeping your habits healthy. It lets you control everything about your health: athletic activity, eating habits, water consumption, etc.