They're fruits that grant the user some type of ability after eating. Luffy, Chopper, and Brook all ate one. So did that "god".

[sandwiches finished~! Next he'll lightly toss the fruit in that sauce of his. Biscuits that are in the oven will be done in another minute or two]

Only one fruit of each kind exists at a given time, so as Nami-san said, only he can turn himself into lightning.