Overlay mortars are prepared for use by mixing cement product formulations with water. The resultant hydration reaction creates the binder which (a) keeps the mass together and gives it body and mechanical strength, and (b) develops adhesion bond to the substrate.
However, porous surfaces are “thirsty” and absorb water from the freshly-mixed mortar at the interface, possibly leading to incomplete hydration of the cement where the bonding is expected to occur, and causing weak adhesion. This problem is prevented by pre-wetting the substrate to “saturated, surface-dry” (SSD) condition, which means all the capillary pores are filled and will no longer draw water from the applied material but the surface is free from standing water.
Rightly, SSD condition can only be assured by soaking the substrate and mopping up or sweeping away all excess water afterwards. If soaking is not practical, at the very least, hosing-down liberally is recommended. You can perform a simple test to check that pre-wetting is properly done by placing your palm on the substrate; it should feel damp without wetting your hand. Apply the overlay mortar immediately.
(Reference must be made to the manufacturer’s technical data sheet for instructions on use of particular products.)