can I use barley instead of rice

In many or most dishes, yes.
Barley will give you a small kernel of cooked starch with a more nutty taste than (white) rice. Can be used as base when serving protein, in soup, frittatas, puddings, etc.
Not exactly the same nutritional profile, but both are composed of around 90% carbs, 7-10% protein, remainder fat.

For what???

William, what are you trying to make? If you can provide some more details/information, I'm sure the folks here will have some great advice.