1) Ground cloth 2) Piece of cloth 3) Piece of material 
A sheet of waterproof material such as woven PE or PVC that is on the ground under the tent and intended to keep out moisture
[n] - a waterproofed piece of cloth spread on the ground (as under a tent) to protect from moisture
ground cloth noun a waterproofed piece of cloth spread on the ground (as under a tent) to protect from moisture
Also referred to as a footprint, this keeps your tent free from dirt and moisture and adds a level of insulation.