Fill handle
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 below picture indicated by the red arrow.
If you enter the number 1 in cell A1 and the number 2 in cell A2, you could extend that numbering sequence down through any number of cells you desired by selecting both cells, then clicking on the small black box, the Fill Handle, 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 will make column A have 5, 10, 15, 20, 25, etc., with spaces in-between each cell.
Also see: Paint program, Region fill