Well, you see, that's simple
The earth is one of the few planets with its own, sustainable atmosphere. The sun is the earths #1 source of energy, in fact almost ALL heat, thermal, and light energy comes from the sun. So, if the sun is a big firey ball of gas that spits out heat like eminem spits out lyrics, why haven't we all been burned to a crisp?
Without the sun, earths temperature would be at or very close to absolute 0. However, the earth ABSORBS some heat from the sun in it's oceans, atmosphere, and land. The sun sends out rays of heat and light to us, and the earth's atmosphere sends it all right back, just as infrared light. The earth stores what it needs from the rays, and sends everything else back.
This is what keeps the earth from overheating from the sun.
~Hope this helps!
