The small green square at the bottom-right corner of a selected cell used to copy data or extend a series.
Try it with AIThe fill handle is a feature that allows users to quickly copy cell contents, formatting, or patterns to adjacent cells. By clicking and dragging the handle, Excel automatically replicates the data or continues a logical sequence, such as dates or numbers.
Type '1' in cell A1 and '2' in A2, select both, then drag the fill handle down to automatically populate the column with 3, 4, 5, and so on.
It is likely disabled in your settings; go to File > Options > Advanced and ensure 'Enable fill handle and cell drag-and-drop' is checked.