Why are greenhouse gases important?

Greenhouse gases are unique atmospheric gases in the way they interact with energy.  They allow light energy traveling from the sun pass directly through them to reach the surface of the earth.  Once that light hits the Earth, it becomes heat energy and a portion of that radiates back out towards space in the form of infrared energy.  Unlike sunlight, infrared energy is absorbed by Greenhouse gases and a portion of that energy is reflected back to the surface of the Earth.  The overall effect Greenhouse gases have is like a layer of insulation keeping a portion of Earth's heat from radiating out into space.