Insulin is a hormone made by beta cells in the pancreas. Its main job is to let glucose (sugar) move out of the bloodstream and into your cells, where it's used for energy or stored for later. Without enough working insulin, glucose builds up in the blood instead of reaching the cells that need it.
Why the body needs it
Every time you eat carbohydrates, your blood glucose rises. In response, the pancreas releases insulin, which acts like a key: it unlocks cells in muscle, fat, and liver tissue so glucose can enter. Insulin also tells the liver to store extra glucose as glycogen, and it slows down the liver's own glucose production once levels are high enough.
What happens without enough insulin
In type 1 diabetes, the immune system destroys the beta cells that make insulin, so the body produces little to none. In type 2 diabetes, the body still makes insulin but cells respond to it poorly (insulin resistance), and over time production can decline too. Both situations can lead to chronically high blood glucose, which is why many people with diabetes take insulin as medication - injected or pumped in, since it can't be taken as a pill (stomach acid breaks it down before it can work).
Quick fact
Insulin was first successfully used to treat a person with diabetes in 1922. Before that, a type 1 diagnosis was almost always fatal within a year or two.
Natural insulin vs. insulin medication
The insulin your pancreas makes and the insulin in an injection pen work the same way biologically - modern insulin medications are manufactured to be structurally identical or very close to human insulin, using lab-grown bacteria or yeast rather than being extracted from animals (as was standard until the 1980s).