"WR" is Water Rate, also known as "Steam Rate". It is the amount of steam required for a given unit of work. The units in your example are actually lb/hp-hr, NOT lb/hp/hr.
The expression "Water Rate" is from the perspective of the boiler or more specifically the feedwater, and not the turbine. In any case, the expression persists.
There are different specs that sometimes appear that all look like "water rates". There might be TWR (or TSR), AWR (or ASR), and perhaps others. These are THEORETICAL Water/Steam Rate, and ACTUAL Water/Steam Rate respectively. The Theoretical rate corresponds to the theoretical minimum steam required with an isentropic expansion of the steam.
Regards.