What is homeostasis?

Homeostasis is the body's ability to keep the body's internal environment at a condition of equilibrium, in response to any external changes. This includes body temperature, salt concentration, water concentration etc.
For example if it's too hot externally, your body will adapt to ensure that your internal body temperature also doesn't get too hot as it can be dangerous. Thus you will do things such as sweat and your blood vessels will dilate to let out more heat, etc.