What Keeps a cell reference constant when copying a formula or function?

If you want to copy a formula from one cell to another (or fill down) without Excel changing the cell references automatically, you'll need to write the cell references with dollar signs included for absolute referencing.

Eg: the reference '$D3' locks the reference to column 'D' but allows the row to change when the cell is copied.

Eg: the reference 'D$3' locks the reference to row '3' but allows the column to change when the cell is copied.

Eg: the reference '$D$3' locks the reference to column 'D' and row '3', so the cell reference cannot change at all when the cell is copied.

While you are typing in a cell reference, pressing the 'F4' key will cycle through the combinations for you, so you don't need to type the dollar signs yourself.