In Google Sheets, a cell reference is a cell address of another cell or range of cells. It is useful when you use formulas.
For example, you have =sum(A1:A2) formula in the above mentioned cell A3. It means that sum the values contain in cell A1 and A2.
Both A1 and A2 are being used here as cell references of cell A1 and A2 for the formula.
Google Sheets use relative cell reference as default. It is named “relative” because the reference is used with reference to location of a cell or a range of cells. In our above mentioned screenshots (example), relative cell references are based on location i.e. cell A1 is one row up and cell A2 is 2 row up to cell A3.
When you copy the formula having the relative reference and paste it in the same row. The reference in the formula will change with respect to location.
Absolute cell reference
You may retain original cell reference by making it absolute with the ($) dollar sign. For example, to make the cell reference absolute select the formula in the cell A3 and press F4. The ($) should be shown in with the cell references making it to absolute cell references.
Now when you copy and paste the formula to cell B3, the formula will retain the original cell references.