Insulin is a hormone that lowers the level of glucose (a type of sugar) in the blood. It's made by the beta cells of the pancreas and released into the blood when the glucose level goes up, such as after eating. Insulin helps glucose enter the body's cells, where it can be used for energy or stored for future use.
In diabetes, the pancreas doesn't make enough insulin or the body can't respond normally to the insulin that is made. This causes the glucose level in the blood to rise.