In math, the "sum" of two numbers means to add them together. In a spreadsheet, the "SUM" function means something more like "compute these numbers." It can contain many different mathematical operators the most common being:
+ to add
- to subtract
/ to divide
* to multiply
: "through" or add all of these cells together
The SUM function is used in almost every sheet that includes a calculation:
=SUM(A1+B1) adds cells A1 and B1 together
=SUM(A1*B1) multiplies cells A1 by B1