Before the apocalypse, Camp Cackleberry was a camp, possibly for children.
At some point during the outbreak, the camp became home to a community of survivors, including Max, Annie, and Dylan. It eventually came under attack by a herd of walkers, but the survivors won the battle, killed the walkers and repaired their fence.
After the battle, unaware that the walkers were radioactive, the survivors burned their bodies. As a result, the radiation was released into the air and they developed terminal radiation sickness. The ill survivors barricaded themselves inside of cabins with signs warning that sick people were inside and to stay away. They eventually died of radiation sickness and remained as walkers trapped inside the cabins. Though the adults died, Max, Annie, Dylan and a group of kids managed to survive and escape.
After encountering Max, Annie, and Dylan, John Dorie and June spotted a sign for the camp which they recognized as being the same as the one on the van Annie was driving. Suspecting it might've been their home, the two headed into the camp where they found the aftermath of the battle and the zombified residents. Seeing the dosimeters on the burned walkers, the two contacted Morgan Jones, Alicia, and Grace who had warned them of radioactive walkers. After confirming it with a Geiger counter, Grace realized that the residents would've made themselves sick by burning the bodies and was saddened to find them reanimated in the cabins. Feeling that it was her responsibility as Grace blamed herself for the radioactive walkers, she entered the cabins and put everyone down. She later warned the others to leave as the campground was not safe due to lingering radiation.
John and June are later attacked by Dwight due to the van they took from the campground. Dwight eventually reveals that the van was what his missing wife Sherry was driving the last time he received a note from her. The van contains no further clues and John and June avoid telling Dwight the fate of the camp's residents, insisting that Sherry might still be alive somewhere. John later determines that the car is actually not the correct vehicle that Dwight was looking for as the VIN number doesn't match the registration, further suggesting that Sherry is still out there somewhere.
After rescuing Max and Annie from a group of walkers, Morgan and Alicia were confronted by the other kids before they defused the situation. Morgan quickly realized that they were the children of the people they had found at Camp Cackleberry which they confirmed. The kids refused to leave the area even though there was nothing left there for them which Morgan understood given his own experiences with Jenny Jones and Duane.