Tall grasses are resource objects placed randomly on meadows, mostly on the outer ones. Cutting it with a scythe sometimes yields one unit of food, wood or gold. Rabbits feed on it to multiply their numbers. With the Farming upgrade, you will always get 1 food as loot.
A minion with a scythe will automatically move to tall grass to cut it. One swing is enough to gain loot, if there is any. Like other resources, it will regrow when a cloud comes closer to rain over the area or after some period of time.