It is the Fill Handle, which is most commonly used for copying values in a column or a row by dragging on it. It has a lot of uses in the ways it can copy and fill cells with values, often doing things like incrementing them or filling lists such as the days of the week or months of the year.
The small black square in the lower corner of an active cell in Excel is known as the Fill Handle. You can use it to quickly fill data into adjacent cells by dragging it.
thin black cross
The fill handle is the small black square located in the lower right corner of the heavy border around the active cell. The active cell cannot be identified by a block arrow displaying in the center of the cell.
Fill Handle
It appears as a small black square in the bottom right corner of the box around a cell or cells when you have a cell or cells selected.
The little square in the lower right corner of a cell is called the fill handle.
At the bottom right corner of the active cell there is a small black square. By putting the mouse on that and then pressing and holding the right mouse button you drag across a formula.
When you add a comment to a cell, there is a small red triangle in the top right corner of the cell.
fill handle
It is normally the Fill Handle.
It is the fill handle, enabling you to drag down a cell with a formula or for extending pre-defined lists. For example: if you type Sunday into a cell and press return, and then use the fill handle to fill down, it will automatically put in the other days of the week. You can do the same for Months.
To copy a function to another cell, you can simply click on the cell containing the function, then use the keyboard shortcuts Ctrl+C to copy, and Ctrl+V to paste it into the desired cell. Alternatively, you can also click and drag the fill handle (a small square at the bottom right corner of the cell) to the cell where you want to copy the function.