Refer to a cell
Ability of Excel to link to another cell in the same or a different worksheet is the secret of its power and versatility. Just type = in a cell and select the cell you want to refer to. For example to refer to cell D4, type = in any cell (say J21), select D4 and press Enter; the value 53 will be displayed in cell J21 as well.
Alternatively, you could simply type =D4 in cell J21 to get the same results.
Note that if you now change the value of cell D4, the same change will be reflected in cell J21. Thus if you need to update cell D4 you would not need to update all cells where that cell is referred to.