A bottle filler is a tube with a stopper on the end used for filling bottles.
The bottle filler or 'little bottler' is attached to the bottling bucket and the tap is turned on. The stopper prevents the beer from escaping. The bottle filler is put into the empty bottle, which pushes up the stopper, allowing the beer to fill the bottle. When the bottle is filled, the bottle filler is removed. The volume taken up by the filler is approximately the amount of headspace needed for the beer.