A Damp-Proof Membrane (DPM) is a barrier used to protect a solid floor from moisture, or 'rising damp'.
Damp-Proof Courses (DPC) provide a similar function for walls.
These membranes do not provide absolute protection from moisture, but is generally sufficient to stop dampness. This occurs as a result of capillary action, as water rises up through building foundations, which, if untreated, can lead to dark spots on walls and surfaces, and cracks.
This is a particular problem for some older properties where no damp-proof course or membrane has been installed whatsoever.