Formulas let you define data in a column as a function of data in other columns. For example, if you record the length (L) in one column and width (W) in a different column, you can use the formula W * L to automatically calculate the area and store the product in a third column.
Suppose you have data in C1 and C2. You want the sum of these two columns to be in C3, and the product of the two columns to be in C4. You want Minitab to automatically recalculate C3 and C4 whenever the data change in either C1 or C2.