High blood glucose damages the function of pancreatic beta-cells. Hence these cells cannot release enough Insulin hormone, leading to the progression of type 2 diabetes.
Blood glucose is the main source of energy and comes mainly from food intake. Insulin, a hormone made by pancreatic beta cells, helps glucose get into cells to be used for energy.
In type 2 diabetes, the body doesn't make enough insulin or doesn't use insulin well. Too much glucose then stays in the blood, and not enough reaches the cells.