The snake represents the "devil" or evil and the apple represents the fruit of a tree. Adam and eve were told not to take the fruit of the tree because they didn't need it. However they had the temptation to take the fruit of the tree because they were told they shouldn't take it. Just like Macbeth when one of there prophesier came true. So he is wondering if they should trust what they say. Banquo says
This example of equivocation is similar to the armed head that warns Macbeth of Macduff, as it simply tells Macbeth to be wary of MacDuff rather than tell him to be cautious of him as he will kill him.