In Excel the data are contained in ‘cells’ that are referenced using a grid numbering system similar to that used on maps.
You select a cell by clicking on it. Cell ‘D5’ is selected in the illustration below. It contains the value ‘8’. The formula bar shows the formula that led to that value, it is the addition of the contents of cells ‘C3’ and ‘D3’:
5 + 3 = 8
If you change the values in any of the source cells ('C3' or 'D3') then the value in 'D5' will automatically update: