The Autosum button automatically builds a formula which adds up all the figures in a range on a per column or per row basis.
To add figures automatically:
Select a range with either a blank row at the end or a blank column at the right. If the blank cells are at the bottom of the range, the numbers in the columns will be added; if the blank cells are at the right, the numbers in the rows will be added.
Click the Autosum button on the toolbar.
A formula will be created in the blank row or column at the end of the range, which adds all the numbers in the column or row and displays the result. If you click on one of the cells, you will see the formula on the formula line.
If you click on the Autosum button without making a selection first, it simply enters SUM() in the formula line ready for you to supply the parameter.