The home screen shows your progress towards your daily goal. Your progress is always measured in the same units as you goal. For example, if your daily goal is 1 Liter and the container you have selected is measure in ounces, WaterWorks autoamtically converts the number of ounces you've recorded to Liters when displaying it here. If you decide to change your daily goal to be measured in gallons, your progress will be converted to gallons instead.
Clicking the "Record It!" button will record the selected container's amount. The selected container is shown near the bottom of the screen, under the image of the bottle.
To change the selected container, tap on the image of the bottle.