Two CDs purchased in February cost $12.99 and $16.99. The total of these two values is the CD expense for the month.
You do math in Excel by typing simple formulas into cells. Excel formulas always begin with an equal sign (=). Here's the formula typed into cell C6 to add 12.99 and 16.99:
=12.99+16.99
The plus sign (+) is a math operator that tells Excel to add the values.
If you wonder later on how you got this result, the formula is visible in the formula bar 
near the top of the worksheet whenever you select cell C6 again.
