When you clip grass or other plants, you mow it. If you realize your front lawn is taller than your dog, it might be time to mow it.
People with grassy lawns usually mow them throughout the growing season, keeping them neatly trimmed and green. Farmers mow some food crops and hay as well, when they're ready to be harvested, although they use different tools to do it, including a curved blade called a scythe. An old-fashioned meaning of mow is "a stack of hay," or "a place where hay is stored."