Repairing hollowed out hole in hardwood floor
You should buy a colored wood filler that most closely matches the color of your floor and pack it in there to fill. Carefully sand the filled area, hit it with additional stain if necessary, then varnish the patch.

Note: prior to patching you should investigate the cause... cuz it sure looks like termite damage. You could treat the area with local termite killer spray prior to patching but if you have widespread or structural infestation you may need a more invasive technique (tent the house).