When you consume sugar, your body converts it into glucose. Glucose is what provides our body with energy to help us function. Before you think the answer is to continue eating loads of sugar so your body can turn it into glucose and therefore energy, think again. If you consume more sugar than your body needs, it will store the excess glucose as fat.
Without insulin (the key), the glucose (sugar) remains in the bloodstream and cannot enter the cells, leading to high blood sugar levels. This is what happens in diabetes, where either the body doesn't produce enough insulin (Type 1 diabetes) or the cells become resistant to insulin's effects (Type 2 diabetes).