Excel has Functions which assists with creating efficient formulas.
Rather than entering in a formula such as =A1+A2+A3+A4+A5+A6 into cell A7, a Sum function can be user per the above example.
The Sum function adds together a range. The range in the above example is A1 to A6 (Or A1:A6).
Note: You can sum vertically by columns and you can also sum by rows.
By clicking on the auto sum icon, you can automatically insert the sum function in the formula.
Other functions can be found by following Insert, Function.
More functions will be explored further in the Intermediate and Advanced user guides