On the main screen you can see the daily dashboard. 

Your daily calorie budget is labeled 'Kcal Goal'.

'Locked' are the calories that should be burned through physical activity for the present day. They can be unlocked by exercising, using step counter or they will be transferred from Health (default Apple application on iPhones).

'Used' label refers to the calories from the food that you have logged today. 

'Currently available' calories in the chart are the calories that are available for nutrition without taking physical activity into account. 

The formula is: Target kcal - Locked - Used = currently available.