The status bar appears below the grid. When used in combination with the range selection it displays aggregation data about the selected range giving the following: average, count, min, max, sum.
If you have multiple ranges selected (by holding down ctrl while dragging) and a cell is in multiple ranges, the cell will be only included once in the aggregations.
If the cell does not contain a simple number value, then it will not be included in average, min max or sum, however it will be included in count.
In the grid below, select a range by dragging the mouse over cells and notice the status bar showing the aggregation values as you drag.