Grass means lawns and mowing and it’s what horses and cows eat. That’s about all of what most people know about grass. There is an old phrase, ultimately from the Old Testament, that all flesh is grass, meaning simply that animals eat grass and people eat the animals, so essentially people are made grass.
That phrase is not heard much these days, but remains true. Most of the people on our planet are made of grass, either directly or indirectly. It varies by region, but the main foods of humanity are grasses. These include oats, wheat, rye, barley, rice, millet, and corn (outside the US, often called maize). Some are eaten directly, and some grains (corn in particular) are used primarily to feed livestock. But ultimately humans in the 21st century are just as dependent on grains as their ancestors were in the 11th century.
Grasses turned into crops are termed “cereals,” named after the Roman Ceres, goddess of the harvest. There are a few crops termed “pseudocereals” that are not derived from grasses, although prepared much the same way. These include amaranth and quinoa, important only in a few regions.