Carbohydrates are a primary source of energy for cells. Carbohydrates are carbon, hydrogen and oxygen. Carbohydrates are made up of fiber, starch and sugar.
Simple carbohydrates release glucose aka sugar into your system quicker than complex carbohydrates. Simple carbohydrates can lead to sugar spikes and even insulin resistance.
Simple Carbohydrates are made up of one or two sugars, which are released and digested easily/quickly with intent to spike the blood sugar.
Simple Carbohydrates are listed as refined sugars which has zero nutritional value. I general, you should replace simple carbohydrates with complex carbohydrates.