Excel's fill handle is a small rectangular control that appears whenever a cell or range of cells are selected. When the cursor is moved over the fill handle, it turns into a cross symbol, which can be dragged to fill in more cells.
The behavior of the fill handle varies depending on the content of the cell or range selected:
When the selected range contains formulas, the formulas are copied to new locations and references are updated.
When the selected range contains text or numbers with no recognizable pattern, values are copied.
When the selected range contains text or numbers with a recognizable pattern, the pattern is continued.
When an adjacent column contains values, double-clicking the fill handle will extend values down the worksheet to match, following the behaviors described above.