Demand means "an urgent request," like your demand that teachers give no homework on the weekend, or the act of making the request — teachers who demand that the work get done, even if it's the weekend.
Demand comes from the Old French word demander, meaning "ask, make inquiry." A demand is a lot stronger than that, however. When you demand something, you want action now. A demand can also mean "to require" like cold weather that demands warm coats and boots. Demand2020欧洲杯时间 is also an economic term, meaning "the amount of an item that can be sold," like the demand for those snow boots: high in winter and nonexistent in summer.