A formula field calculates a result from other values. Users edit its inputs rather than typing into the calculated result. Arithmetic, conditional logic, text, and date expressions all need a defined return type. A formula that looks right on one record can still fail on a blank or zero input.
Percent fields require special care: a displayed 60% is 0.6 inside a Salesforce formula. Multiply Amount by Probability directly for a weighted amount. Guard a denominator before division, and decide whether a missing value should remain missing or become a stated default. The checker evaluates your formula on independent records, so hardcoding the visible result doesn't solve the task. Use the data's meaning to choose the expression.