Earth is a planet which was settled on by Kobol's Humanoid Cylons following their exodus.
William Adama: What have I done? All the people I've sent to die, for what? For what?
Lee: For Earth.
Adama: There is no Earth. It's a frakking joke. There is no Earth.
Lee: Okay, Dad, listen to me. Listen to me! Pull it together.
--Lee trying to sober up a drunken William Adama.
The first Earth had long been destroyed before the Fleet could reach it. Radiation measurements of the water and ground showed that the planet had gone through a nuclear event long ago, and that it was once inhabited by humanoid Cylons and mechanical Cylons before this event. The planet was tainted with destruction, debris of fallen structures as far as the eye could see. The members of the Fleet declared the planet uninhabitable, and went back to their ships in search of another planet to call home.