Grass is a plant species, commonly used to decorate a person's lawn. A person's lawn is usually cut with a lawn mower. Grass is used for a number of things. On farms, Cattle and horses both eat grass, as do many other animals, who eat it as food. As such, it is a widely cultivated agricultural product, and its' seeds, which are known as grain are used to make a variety of foods, such as cereal, bread and pastry.
When the very small Krim family landed their spacecraft in Marvin's backyard, they thought they had landed in a jungle, but in reality, they were so small compared to the grass that it only appeared that way.