Hormones affect many different body functions like growth and creating energy. They're what caused your body to go through puberty. When they are not working properly for whatever reason, several problems can occur. Your menstral cycle could be disrupted, you could gain weight or develop diabetes and such. If you think you might have an hormonal imbalance, it's important to visit your doctor. You might want to keep a daily diary of your symptoms, and include other details like when your period starts, how long it lasts etc. That will help a doctor a great deal in determining where the likely problem is.

In short it is when one or more hormones are either too low or too high causing the overall hormone levels in the body not to be balanced.