The Dead Zone is a void of darkness created by Garlic that sucks in living things and it traps them inside forcing them to float through endless blackness. Garlic's son Garlic Jr. has opened the Dead Zone twice, but in both times, he was sucked into it and the void closed up. When sealed the first time, the seal shattered into various pieces, which all have Garlic Jr. knocking futilely in hopes he might escape.
During the battle at his castle-like fortress he opened it in an attempt to suck in Goku and Piccolo, knowing that despite his immortality, he could not beat them. But the young and inexperienced Gohan was able to resist its suction and knock Garlic Jr. into the Dead Zone with his hidden power and the fortress was left in ruins and soon abandoned. Garlic Jr. initially did not care, since he felt that his being immortal would have him survive the Dead Zone, but then realized that he is sealed in.
Garlic Jr. remained in the void for about 3 long years until the Makyo Star got close enough to Earth, allowing him to escape from it. In the Garlic Jr. Saga, Garlic Jr. reopened the Dead Zoncountretaliation to his dark plans being foiled (Kami and Mr. Popo had broken his curse on the people of Earth using the Sacred Water which counteract the Black Water Mist). Piccolo, Krillin, Kami, and Mr. Popo were nearly sucked into the Dead Zone, but Gohan destroyed the Makyo Star with an intense energy blast, draining Garlic Jr.'s fearsome strength and causing him to be sucked into the Dead Zone once again.
By the end of Dragon Ball Z (though it is never referenced following the Garlic Jr. Saga, save for maybe Piccolo's line of the Lookout's being completely ruined "again") he still lives, immortal, and is still trapped inside the Dead Zone, never to escape, and very likely regretting his immortality.