When using SUM in Excel what does it mean?

It gets a total of the values in the cells that are selected or values that are listed in the function. It is a very commonly used function as you need to do a lot of adding in a spreadsheet. There are lots of ways of using it. You could use a cell range like this:

=SUM(A2:A10)

You can use individual cells:

=SUM(A2, A12, A14,B10)

You can use actual values:

=SUM(10,34,20)

You can use it mixing any of those:

=SUM(23,A3:A65,C10,D40)