You've probably heard the term "blood sugar" and heard people talk about whether theirs is high or low. "Hyperglycemia" just means high blood sugar; hypoglycemia means low blood sugar. Your body works best when your blood sugar levels stay within a certain range. Too low, and your energy and ability to think clearly will crash; too high, and all kinds of things start to go out of whack. Long-term high levels -- which happens when you have diabetes -- can do permanent damage in your smaller blood vessels: feet, hands, and eyes.
Here's a 9-minute video from Khan Academy which explains -- in a very simplified manner - what happens to your glucose (blood sugar) levels after you eat. And how the blood sugar levels in a diabetic differ from those in a healthy individual.
image source: girl eating sweet