Carbohydrates are a source of energy. The more complex the carbohydrate, the longer it takes to release the energy. 1 gram of carbohydrate contains 4 calories, so it’s worth knowing how that energy is released.
Carbohydrates are usually split into two main categories, simple carbs (or sugars) and complex carbs (or starches). This is a good general rule, though there are a few bits worth getting your head around.
Carbohydrates are built from units of sugar (saccharides), the longer the chain of units, the more complex the carbohydrate. To use the sugar your body needs to chop up this chain. This means that complex carbohydrates release their energy slower than simple (short chain) one.
The longer it takes to turn it into sugar, the more complex (and probably better) the carbohydrate.