In The Lord of the Flies, the boys mean that the island is a good island because there is food, water, and a means to make shelter for them to survive. It is also good because they are British boys who believe that they will be self sufficient until help arrives. They will be able to organize themselves, figure out what to do to make the best of their situation, and have fun besides. By itself, the island is not good or bad because an island is just that--a piece of land surrounded by water. Technically, the island could be bad if it had no food on it or any way to create a shelter, or it could be bad if there were people on it before they arrive who would try to kill them. The boys themselves will make the island bad by their behavior and descent into savagery.
After the plane crashes, Piggy and Ralph are talking on the island. In this context, Piggy reminds Ralph that no one know where they are. So, a rescue will be difficult and in the least a rescue will take time. Ralph, then, says:
“So we may be here a long time.”
Nobody said anything. He grinned suddenly.
“But this is a good island. We–Jack, Simon and me– we climbed the mountain. It’s wizard. There’s food and drink, and–” “Rocks–” “Blue flowers–”
The island is good partially, because it is inhabitable. The island has the basic ingredient - food and water for the boys to survive.
Jack also chimes in and adds that there is also meat on the island - pigs. He says:
Jack held out his hand for the conch.
“There’s pigs,” he said. “There’s food; and bathing water in that little stream along there–and everything. Didn’t anyone find anything else?”
In conclusion, the island is able to support life until a rescue comes.