To Have a Healthy Lawn, you need three things (mainly):
No Weeds
Nutrients
Consistency
No Weeds- Weeds will not only decrease the aesthetic appeal of the lawn, but will also choke the lawn, deprive it of nutrients, and starve it of the nourishment the grass needs to survive and thrive.
Nutrients- How can a plant grow if it doesn't have anything to grow with? Water, Sunlight, and Fertile Soil are the things grass uses to grow and be healthy. By watering correctly and fertilizing often, your turf can become thick, green beautifulness.
Consistency- Grass is a living organism that adapts to its environment. By mowing, watering, fertilizing, etc. consistently, your lawn can get used to its situation and will be malleable. With time, we can cut the lawn shorter and shorter until it's perfectly green and spongy like a golf course!