On the right side of the status bar are several squares. One of the squares shows information about the cells you have selected. The default information is SUM, but you can change it be right-clicking on the box and choosing another operations.
I use this all the time. The best time to use it is when you’re looking at a spreadsheet with someone who doesn’t know it exists. You can select a range of cells – and they don’t have to be contiguous – and quickly tell the onlooker the sum. When they ask you how you added those numbers up so fast, the correct response is “What? You can’t add that fast?”. Never tell your secrets.