In MS-Excel, cell B9 contains the value 3 and cell B10 contains the value 6. You select both cells and drag the fill handle down to B13. The contents of cells B11, B12 and B13 will be

Selecting 3 and 6 sets a step of 3, so filling down gives 9, 12 and 15.

Excel reads the two selected cells as a series increasing by 3 each step.

B11 continues the pattern as 6 plus 3, which is 9.

B12 continues as 9 plus 3, which is 12.

B13 continues as 12 plus 3, which is 15.

The fill handle extends a detected numeric pattern automatically.