A spreadsheet feature.
It is the very small square you see on the bottom right of the active cell.
If you drag it either across columns or rows from the active cell, then each cell is filled with the value in the active cell. e.g. if the active cell contains '2', drag the fill handle downwards and each cell is also loaded with '2'.
Challenge see if you can find out one extra fact on this topic that we haven't already told you
Click on this link: Fill handle