Tuning is making each string in the piano vibrate at the correct pitch when struck by its hammer. The strings (there are well over 200 of them!) are wound around a tuning pin which is turned by a tuning hammer or tuning lever by the technician.
Tuning is NOT fixing a sticking key or any other problem, although often some minor problems are fixed during the course of a tuning. Most manufacturers recommend that their pianos be tuned twice per year. It seems obvious to say, but a regularly tuned piano stabilizes better and doesn't go as far out of tune between tunings as a neglected piano.
If it has been years since your piano has been tuned, it will very likely require a 'Pitch Raise' before a fine tuning can be administered. It's something like tuning—the same actions are taken—but the main goal of a pitch raise is to re-establish correct pressures on the strings and plate. For more information, go to the Piano Technician's Guild article on pitch raising.
For more information on piano care in general, go to the Piano Technician's Guild website.