In Microsoft Excel, a fill handle is a feature that allows the user to extend (and fill) a series of numbers, dates, or even text to a desired number of 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 to the right.
If you enter the number 1 in cell A1 and the number 2 in cell A2, you could extend that numbering sequence down through as many cells as you want. You can do this by selecting both cells, then clicking on 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 - 50 without having to type each number individually.
Another good example of the fill handle, is to enter 5 into cell A1, then 10 into cell A3, drag a box around cells A1 though A4, and then drag the fill handle down as far as you want. This series of actions makes column A have 5, 10, 15, 20, 25, etc., with spaces in-between each cell.