RDH stands for Refrence Datum Height. It is difficult to explain without any figure, but I will try:
The glide slope beam in an ILS is not an absolute straight line which is extending from GS antenna. In fact it is somehow curved in the vertical plane and this bending is more pronounced as the terrain closes particularly over threshold. That is why the extension of avarage of glideslope curve between the distances 1600 and 4500 feet from runway threshold, crosses the threshold at a slightly different height than actual Glide slope beam. Now I will put some extraction from a document mentioning about RDH:
Instrument Approach Procedure designers use the ILS Reference Datum Height to determine the obstacle clearance surfaces of the
approach in accordance with ICAO DOC81684(Pans-Ops). This is whythe reference datum height is quoted as 15m with only a positive tolerance. If the instrument approach procedure is constructed using a reference datum height of 15m, then any increase in RDH serves to increase the safety margin of the procedure.