Discrete means separate or divided. A discrete unit is a separate part of something larger. A room is a discrete space within a house, just as the crankshaft is a discrete part of a car engine.
If something is discrete, it has its own space. An ice cube comes from an ice tray, but it has its own discrete compartment. A student government might have discrete committees for different projects. Don't confuse discrete with its close cousin discreet, which means “appropriately private.” Billionaire Bruce Wayne, for example, is very discreet about his secret life as Batman. You could say Batman is a discrete, or separate, part of Bruce Wayne’s identity. Anything distinct and separated is discrete.