The Harvester's Key reduces the time it takes for a harvester to build.
Once the countdown on a harvester has ended a player needs to stand on the circle in front of it for the harvester to build, that usually takes a long time. With this card equipped it only takes six seconds.
That's all it does.
You can discard the card after but you can't put it back in your active slots during that game. If you are the designated player to get the harvesters built quickly do them all at once then discard the Harvester Key card and fill the slot with another card.