When you eat your body breaks down carbohydrates into glucose (sugar). Carbohydrates are in starchy foods like bread, potatoes and rice. They are also in fruits, some dairy, sugar and other sweet food.
The glucose is taken into your blood stream. Insulin is needed to move this glucose into the cells in your body, so that it can be used as energy. Normally, insulin is produced in the pancreas.
In someone with type 1 diabetes, the autoimmune reaction in the pancreas has killed off the insulin-producing cells. So, they don’t make any insulin and glucose builds up in their blood. We tend to call blood glucose by its other name blood sugar which is easier to remember.