Healthy means having good health. It's the opposite of sick, but also can mean "doing well" in a general sense. You can have a healthy attitude, or be served a healthy portion of food.
Health is one of the most fundamental human concerns. The ubiquitous "How are you?" is often asking after a person's health. To remember healthy think of the proverb: "Early to bed and early to rise makes a man healthy and wealthy and wise."