Barley's a kind of grass, like wheat, that is a good source of carbohydrates for people. Barley is different from wheat mainly in that it will grow with less rain, so you can grow it more different places, and also in that it doesn't taste as good as wheat. So poor people usually ate more barley, and rich people ate more wheat.
People ate barley like wheat as a boiled porridge, or in soup, and they also made barley bread. Barley is also the grain that people usually used to make beer. But people didn't make noodles out of barley.
Because you only ate the seeds from the barley, you had all the rest of the plant left over. The stalks dried into straw, and you could weave the straw into mats to sleep on, or give it to animals to sleep on, or make shoes or baskets out of it, or tie it into bunches to thatch your house. The leftover straw was nearly as useful as the seeds.
For more information about barley, check out these books from Amazon.com or from your library:
Ancient Agriculture: From Foraging to Farming, by Michael and Mary Woods (2000). For middle schoolers, with plenty of information about how farming got started, and how it worked.
Oats, Peas, Beans and Barley Cookbook, by Edyth Young Cottrell (1974).
Famine and Food Supply in the Graeco-Roman World : Responses to Risk and Crisis, by Peter Garnsey (1988). By a leading specialist in Greek and Roman food and farming, and not too hard to read. Especially good on crop failure.