The Surface is what Skyloftians call the land below The Sky. It was said to be almost non-existent, until Zelda was taken away by Ghirahim's black tornado. Soon afterwards it is revealed that Link is destined to go to the Surface to save Zelda. Only a few individuals on the Surface knew the existence of the Goddess Hylia and Skyloft. After a few thousand years, their story was forgotten and became legend. The game's opening sequence tells of a great war (later learned to be led by Demise) to enslave all the races of the Surface.
Tears of the Kingdom
The Surface is referred to as the main land on the ground level of the Overworld. This is opposite to the Sky Islands that float up in the air and can be reached by Recall and other methods. As well as the Depths, which are found deep down below the surface.