Board wages were monies paid in lieu of meals and were paid in addition to a servants normal salary.

The board wages compensated for a lack of meals when the employing family were not in residence, (when the kitchens would not be expected to work and the French chef would be taken to London with the employing family to feed and impress their guests in Town ) Or they were paid when the servant was travelling on family business and was not able to eat in the servants hall.

To quote from J J Hecht’s book The Domestic Class in Eighteenth Century England:

Board wages were money payments disbursed on a weekly basis in place of meals. Usually they were given for only a limited period, such as when a servant was taken on the road or when he sojourned with his employer in London or remained behind while the family was not in residence 
page 153

If the family were not in residence there would be no need to cook mammoth meal sand so the remaining servants would buy their own meals out of their “board wage”, visiting local inns or other servants in nearby houses for their meals.

The principal object of putting servants on board wages was to curb extravagance. “By their profusion in house-keeping” declared Soame Jenyns, “they have compelled us to allow them board wages….. But the system also had other advantages from the employers point of view. It served his convenience and reduced his responsibilities; it lessened the number of potential pilferers to which his household provisions were exposed. 
As above,page 154

If this system was in operation in the Darcy household (some people disapproved of the system for they considered it encouraged licentiousness by encouraging the servants to loiter at public hoses), Mrs Reynolds would have been receiving board wages at Pemberley when Darcy was not resident with his family, in addition to her normal salary.