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.
Minions with scythes 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.