The shield is an energy field surrounding the planet M32-3R1 and protecting it from starships. Any ship that descends too low from space or ascends too high from the planet's surface encounters the shield. Encountering the shield shorts out everything with hitting the shield being similar to hitting a wall. Simply skimming the shield causes it to draw power from the spacecraft and start a feedback loop that would knock out the vessel's main power. Hitting the shield full-on tears the ship apart and instantly destroys it. The shield is deployed and controlled from Ancient ruins on an inhabited island on the planet itself. (SGA: "Death Game")
For millennia, the shield protected the planet from the Wraith before it was eventually discovered by a Wraith Commander that would come to be known to the people of M32-3R1 as the High King. Seeing the massive population as an easy source of food, the High King set himself and the Wraith up as the planet's "gods" and somehow got a Wraith cruiser past the shield to land on the planet, presumably disabling the shield just long enough to land the cruiser before putting it back up. The High King also disabled the planet's Stargate so that no one could dial out.
In 2005, the First Atlantis Reconnaissance Team and Doctor Radek Zelenka visited M32-3R1 on an exploration mission after detecting the energy signature of the shield. As Zelenka explored the Ancient ruins the shield was projected from with Ronon Dex, the team's Puddle Jumper was shot down by the High King's cruiser. Following the shield nearly destroying the rescue team's Puddle Jumper, they came to the incorrect conclusion that the same thing had befallen the Puddle Jumper carrying Lt. Colonel John Sheppard and Teyla Emmagan.
After escaping from the Holy Island with the rescued Atlantis team and around a dozen contestants in the Games of Life, the rescue team's Puddle Jumper came under attack by the High King's cruiser. Unable to outrun the ship, Sheppard engaged in a game of "chicken" between the Puddle Jumper and the cruiser. While the Puddle Jumper dove towards the ocean at the last moment, the High King instinctively jerked his cruiser higher, sending it crashing into the shield. The impact with the shield destroyed the Wraith cruiser, killing the High King and all of his Wraith.
Following the death of the High King and the other Wraith, Doctor Rodney McKay, who had repaired the planet's Stargate, suggested disabling it again. With the shield protecting the planet from space attacks, the people could deal with any Wraith scouts that came through the Stargate as they would then be trapped and unable to return to their hive ships and lead a ground incursion back. Thus, between the shield and the disabled Stargate, the planet could thrive without threat from the Wraith. However, the people chose to leave the Stargate intact and face the potential danger from the Wraith. (SGA: "Death Game")