What are absolute and relative formulae?
When you copy and paste formulae that reference other cells within a spreadsheet, the cell references are automatically interpreted as being relative to the original formula location.
To stop the location of the referenced cell changing when it is copied elsewhere, you should insert $ signs before the column or row (or both). For example, if =$A1 was copied horizontally, the column would stay the same. If the same formula was copied vertically, the row would change.