What happens the night in the forest?
Brown left his "Faith" and entered a journey, a journey that he does not want to take but one which he (we) must (all) take. We have absolutely no way of knowing what "actually" happened ... whether the journey was a physical journey where he actually wound up a strange satanic service in the forest, or whether it was an imaginary journey (as though the first option weren't :)), something he dreamt when he fell asleep in the forest.
The fact that we do know is that he came to see people differently; he came to see them as much more evil than they probably were because his original vision was totally unambiguous: he saw people as either good or bad, but they weren't a combination of each. Since they couldn't meet up with his expectations, he rejected them ... including his faith/Faith. As a result he died a miserable and unhappy man.
So what happened was that a young man was fronted with the ambiguous reality of his friends and neighbors, was unable or unwilling to accept it, and became a miserable man as a result.
The enemy of the good is the perfect.
If the events in the story actually occurred, then Young Goodman Brown found his fellow townspeople, including Faith, at a meeting of witches. The minister and other religious people from the community were present, and try to persuade Goodman and Faith to join them in their worship of evil. Goodman tells Faith to "look up to heaven and resist the wicked one" (104). But he never finds out whether she resists because everyone disappears, and he finds himself "amid a calm night and solitude" (104). The next day, there is no indication from anyone that the events of the previous night had occurred. Young Goodman Brown turns into a sad and stern man.
Did these events really happen? The narrator suggests that they might have been a dream, a way that the evil inclinations in Goodman's subconscious "leaked" into his thoughts in his sleep. This is a question you will have to resolve for yourself!