Naturals refer to those who either entered the Millennial Kingdom period from the Tribulation without dying or were born from parents that came from someone that survived the Tribulation, such as Kenny Williams, the son of Tribulation Force members Buck and Chloe Williams, who was born during the Tribulation.
They differed from the "glorifieds" (those that have a glorified body) in that they could still father and bear children and that they can still age and die, though the aging process is slowed to the point where a 100-year old person would be like a young adult. However, if a natural has accepted Jesus Christ as his Lord and Savior prior to his 100th birthday or has been a believer that came from the Tribulation, then that person would be allowed to live a long life protected by God until the end of the Millennium, where he would be allowed to enter the "new heavens and new earth". If the person hasn't accepted Jesus as his Lord and Savior by that time, however, he will die. Naturals could also be terminated before their 100th birthday if they commit a sin that's worthy of death.
There were a group of "naturals" that formed The Other Light, worshipping Satan and believing that they could amass a big enough army at the end of the Millennial Kingdom to overthrow God and Jesus with Satan as their leader -- a goal that was never realized as that army would be consumed by fire pouring down from the heavens by God.