Question

what is the use of autosum

Answer 1

In Excel, in the menu panel,  there is an option "Formulas". In that the feature Autosum is available.

We have a range of cells that we want to add (always) using a formula, like total of a series of data.  Then click in the cell for total, then on the autosum feature in the menu.  Now drag the box in the cell to enclose all cells that need to be summed up.

you could do that manually by entering   =SUM(A1:A9)   for example. 
Autosum will do it using GUI , which is simpler.

Answer 2

autosum automatically enters the appropriate formula or function into your spreadsheet to sum up the values .

Autosum is an easiest way to sum up the values of a range of cells.