The fill handle is a small, square tab at the bottom-right corner of the active cell or selection in Excel. You can use it to auto-fill cells adjacent to the selected cell(s) with data, formulas, or even patterns. To use the fill handle, click and hold it, then drag it over the cells where you want to replicate or extend the contents of the initial cell. Once released, Excel automatically copies the content of the original cell(s) to the selected cells, adjusting formulas contextually.