Imagine that you are using Excel to help you fill out a timecard by adding up the hours you work each day. To add the hours for Monday, you would type this formula in cell B4:
As you type, the equal sign (=) tells Excel that this is a formula. B3 and B2 are the cell references that refer to the values in those cells. The minus sign operator tells Excel to subtract one value from the other.
Operations within parentheses are calculated first, so Excel subtracts first. Then the asterisk operator tells Excel to multiply the result of B3-B2 by 24, to convert that result from Excel's decimal time system to a 24-hour system. The result of the formula is 9 hours.
If later on you wonder how you got this result, the formula is visible in the formula bar whenever cell B4 is selected. You could also double-click cell B4 to see the formula in the cell.