Regulation is regular maintenance done to the mechanical parts of the piano to insure that the thousands of moving pieces are properly aligned and moving in the correct way. Regulation can be done in a customer's home where sometimes it's necessary to remove the 'action' of the piano to get to all the adjustments. For more extensive work, the action can be taken to the technician's shop without the need to move the entire piano.
Regulation is a necessary part of owning a piano. If Tuning is like putting gas in your car, then regulation is like getting its oil changed and lubed. Reputable piano stores will completely check the regulation of every new piano they receive; sometimes spending hours making sure new pianos are properly adjusted before they're sold. Depending on the quality of your piano and the amount you play it, you will probably need the regulation checked and adjusted every 5-10 years. Not all piano tuners are capable of adjusting regulation and many would rather just tune your piano and leave. Make sure to ask your technician each time they visit if there are any issues with regulation that might need to be taken care of.