'Harvest", a noun, came from the Old English word hærf-est (coined before the Angles moved from Angeln to Great Britain) meaning 'autumn' (the season), 'harvest-time', or 'August'.
(It continues to mean 'autumn' in British dialect, and 'season of gathering crops' generally.)
'The harvest' came to also mean the activity of reaping, gathering, and storing grain and other grown products during the autumn, and also the grain and other grown products themselves.
'Harvest' was also verbified, 'To harvest' means to reap, gather, and store the harvest (or the crop).
People who harvest and equipment that harvests are harvesters; while they do it, they are harvesting.