As you probably know, Excel Fill Handle is located in the bottom right of any active cell or the bottom right cell of a selected range. It's a quite small black square visible in this snip:Small it is, but it's very important to know how to utilize it, as it can be really helpful in eliminating lots of manual typing and saving time in working with data entry... and much more than this. It can be used for filling any range of cells with increments of any choice you want to select.
To utilize the Fill Handle, first you need to fill one or more cells with values (numbers or text, incl. dates and other custom lists). Hover your mouse pointer over the small square until it changes to a small black cross, then you have couple of choices: you can left-click, right-click or double-click (in some situations); click and drag the mouse in the direction you want to fill up the range of cells with series or custom lists that follow your pre-defined pattern.