Fill handle
In Microsoft Excel, a fill handle is a feature to extend (and fill) several numbers, dates, or even text to other cells. In the active cell of the spreadsheet, the fill handle is a small black box at the bottom-right corner, as shown in the image.
Example of using the fill handle
If you enter the number "1" in cell A1 and the number "2" in cell A2, you could extend that numbering sequence down (fill down) through as many cells as you want. You can do this by selecting both cells, then clicking the fill handle (the small black box) with the left mouse button. While holding the left mouse down, drag the mouse down the spreadsheet in column A. If you dragged down to cell A50 and release the mouse button, cells A1 through A50 would now be sequentially numbered 1 to 50 without having to type each number individually.
Another example of the fill handle is to enter "5" in cell A1 and "10" in cell A3. After that, drag a box around cells A1 though A4 and then drag the fill handle down as far as you want. These actions make column A have 5, 10, 15, 20, 25, etc., with spaces between each cell.
The video below shows different examples of how to use Excel's AutoFill feature.
AutoFill, Paint program, Region fill, Sizing handle, Spreadsheet terms