This drawing shows one of two methods of where the waterproofing in your bathroom is located. Most typically the waterproofing is located below the screed (as in the right side of the picture).
Both methods rely heavily on the membrane because tile grout is porous, and the grout, tile glue and tile screed is saturated permanently with normal daily use. A pinhole in the membrane means big trouble, and regardless, capillary action draws moisture to every corner of your bathroom.
Efflock provides another layer of protection by making the porous material water repellent for a drier, less mouldy and efflorescence free bathroom. It can be added to grout glue and screed to help keep water above the tiles rather than soaking beneath the tiles.