An inner track on the Stargate moves and halts in a particular place, allowing a chevron to snap down and back up, storing the symbol in its memory until all the chevrons have locked. Ordinarily seven chevrons are required to lock on to a destination.
Six chevrons determine a specific point in space. The seventh chevron applies as the point of origin, allowing two points in space to connect and generate an outgoing wormhole.
A more uncommon eighth chevron adds an extra distance calculation to the address -- in essence, the equivalent of dialing a different area code. An 8-chevron address connects the Stargate to a gate in another galaxy (such as dialing from Earth to Atlantis, or vice versa), but this requires extreme power consumption.
Approximately three years into the Atlantis expedition, a 9-chevron address was found in the city's database. Earth launched a massive research project on how to successfully dial the address, eventually using it to connect to the Destiny -- an exploratory vessel launched by the Ancients many hundreds of thousands of years ago. The ship travels from galaxy to galaxy and is always on the move, thus the unique, 9-chevron address is needed to reach it, wherever it happens to be at the time.
See also: address
The Fith Race - Under the influence of Ancient technology, Jack O'Neill uses an eight chevron address to find the Asgard.
Rising Part 1 - Daniel Jackson discovers the 8 chevron address to Atlantis and an expedition is organised.
Air, Part 1 - Dr. Nicholas Rush works for two years on the Icarus Project, dedicated to unlocking the secret of the Stargate's heretofor unused ninth chevron. With help from Eli Wallace, the Icarus Base dials the 9-chevron address and evacuates some 80 people to the Ancient exploratory vessel Destiny.