Regulation is the mechanical adjustment of the internal moving parts “to make the piano’s touch and sound consistent across all notes, allow it to comfortably achieve the widest possible range of dynamics, and make the keys responsive to even the most rapid or most subtile motions of the player.” (Wikipedia)

A lot of experience and judgement is necessary to perform a full regulation since both vertical and grand pianos have over 2,000 adjustments that may be done on a single job.

Many pianos can be regulated in the home in a half day ($450) or a full day ($650). Some owners choose to have their pianos receive extensive attention for a week ($1,850) where the action is brought to the shop containing exotic equipment designed specifically for such jobs. On these larger jobs, felt is replaced on the front rail and balance rail if necessary. The pedals are adjusted and the hammers are voiced. Voicing (also called tone regulation) changes the general tone of a pitch by adjusting the felt on the hammers.

Check out this Tech Bulletin with some details about regulation.