It means the situation in reality.... and not in the abstract.

Well, I see Wikipedia says http://en.wikipedia.org/wiki/Facts_on_the_ground "Facts on the ground is a diplomatic term that means the situation in reality as opposed to in the abstract.... It originated in discussions of the Israeli–Palestinian conflict... It is a literal translation of the Hebrew term 'Uvdot Ba'Shetach'.

I am not certain, but I think the original, literal meaning of "facts on the ground" or "ground truth" is to cartography, mapmaking. A map may be compiled from a number of indirect sources--other maps, aerial or satellite photographs, etc. "Ground truth" means what a personal actually sees when they go to that place in person and inspect what is there, on the ground. Figuratively, it means the truth as known to a person who has actually been somewhere or in some situation, rather than knowing it abstractly or indirectly from reports.