Final answer:
The small, black square in the lower-right corner of an active cell or range represents the fill handle in Microsoft Excel. It can be used for copying formulas, filling in a series of numbers or dates, and autofill.
Step-by-step explanation:
The small, black square in the lower-right corner of an active cell or range represents the fill handle in Microsoft Excel. This fill handle can be used to perform various tasks such as copying formulas, filling in a series of numbers or dates, and autofill. By clicking and dragging the fill handle, you can populate multiple cells with the same data or pattern quickly and efficiently.