This story suggests that it is human nature to be both good and evil. No one is completely good and no one is completely evil. Goodman Brown wants to be good, and he swears that after this last night, he will. However, he also wants this one last night to be evil—he knows that going into the woods on this errand is evil and wrong, because he plans to start being good tomorrow.
Once he enters the forest, he sees Goody Cloyse. She is a woman who well-known to him as godly and associated with the church. However, she knows the devil, just as everyone else in the village seems to, as they all are present at the devil's meeting in the woods. Thus, even those individuals who seem the most godly, the most good, also know evil and have committed sins. No one is perfect. We are human.
However, it seems that most people choose to hide their human imperfection, as Goody Cloyse evidently does. This prevents people from realizing that everyone is imperfect and that imperfection is normal. Goodman Brown, for example, hides the evil within him from his wife and from everyone else in the town that the devil meets on the path through the woods. It is this pretending—this hiding of one's true, dual nature—that constitutes our hypocrisy.