Concrete base is a rock based material placed and levelled under the polythene prior to the form work and reinforcing then concrete. 

See attached photo shows the base, which is crushed rock and is around 50mm to 100mm thick. The need for a smaller aggregate with rounded edges, so the perforation to the polythene is avoided. If a raft slab is being used the use of rock material is not used packing sand takes it's place to form the pods.