What is homeostasis?

Homeostasis is defined as the maintenance of a constant internal environment in the body. 

An example of this would be the blood glucose level:

After a meal, a person's blood glucose level would be high. This stimulates the beta cells of the islets of Langerhans in the pancreas to secrete insulin, an enzyme that converts glucose to glycogen to be stored in the liver. This then reduces the blood glucose level back to the norm. 

On the other hand, when a person's blood glucose level is low, such as after exercising, the alpha cells of the islets of Langerhans secrete glucagon, an enzyme that is responsible for converting glycogen to glucose and releasing it into the bloodstream, in order to increase the blood glucose level. 

This is an example of negative feedback (an important regulatory mechanism in the body).