Greened out are the fields that are disabled for editing. The field auto-calculates based upon information entered elsewhere. For example, if you enter your Strength, it will auto-calculate the modifier for you. If you check the Save Prof box, it will apply your Proficiency Bonus to it, which is calculated when you enter your class levels. If you have 3 levels of fighter, 1 level of wizard on the Class tab, it will auto-calculate the Overall Level on the Core Stats tab, which then auto-calculates the Proficiency Bonus. And around and around it goes.