An expanding reference (or expanding range) in Excel defines a range that expands as a formula is copied down or across cells. This is done by "mixing" absolute and relative references – making the first cell an absolute reference and the last cell a relative reference.
In the example shown the formula in D4 is:
As this formula is copied down column D, it changes as follows:
=SUM($C$4:C4) // D4 =SUM($C$4:C5) // D5 =SUM($C$4:C6) // D6
At each new row, the range is expanded to include one new row. As a result, the SUM function calculates an accurate running total.