To simplify it, the cell reference is relative to its location. So, when the cell reference is relative, it automatically changes when you copy it or move it. Similarly, if you check other cells you will notice that D3 references B3*C3, D4 references B3*B4 and so on. Select any of the automatically filled cell (D9 in our case) and check the formula in formula bar.

When you press enter and drag the fill handle, you shall notice that the data has been filled automatically.