A reference to the forbidden fruit in the Garden of Eden, which God forbade the first humans, Adam and Eve, from eating.
The serpent tempted Eve to eat the fruit (there is no mention of an apple in Genesis), and Eve in turn convinced Adam. After eating the fruit they gained knowledge of Good and Evil, and tried to hide their nakedness. God was furious when he saw what they had done, and threw them from the paradisical garden. This is the ‘original sin’ that all humans are tainted with when they are born, according to Christian theology.