If we're talking about the hypothetical of cutting down my insulin, either by rationing it or stopping it entirely, it means things like becoming blind due to diabetes-related macular problems, or diabetic retinopathy. It means neuropathy, which starts with a tingling and numbness in your legs that end up turning into excruciating pain. It also means there's low blood circulation in your limbs, so you're more susceptible to injuries and cuts. As you don't feel those injuries, they fester. Those complications end up leading to amputations.
Other effects are long-term kidney damage and long-term liver damage. All that sugar in your system has to be flushed out somehow, so your kidneys and liver end up working overtime to get that glucose out of your system.