Although it is hard to nutshell YGM, you can summarize the plot as a man's journey from good through evil, the question of WHAT is good and evil, and the realization of one's own feelings towards it.
The setting is in Salem, where Goodman is about to go out at night to take care of something, to the distress of his wife Faith (that is a symbol, moving from "faith)- Faith is a prudish woman, all the way down to what she wears (pink ribbon on her hair, a symbol of innocence).
When he goes anyway, he meets a stranger in the forest with whom he walks along and whom he suspects of being evil.
We know that this stranger symbolizes sin, or the devil himself, and here's Goodman is walking with him. Along the journey, the man tells him how he's known Goodman's family and other things that start freaking out Goodman, and he starts deciding to go back to Faith. But, its too late b/c in the end of the road there is a Satanic-like ritual taking place in which he sees his dead father, and even Faith herself partaking in it. When the stranger tells him that "everyone" is evil, Goodman loses it and starts screaming, and the entire moment faded in front of him as if it had been an illusion.
However, when he is back in town, in Salem, he doubts everyone, shuns away Faith, and distrusts all the people he once thought were good. He sort of lost his mind, and died a recluse.
So, the symbols include Faith (her name), her pink ribbon, the night to day change of scenery (shadow vs. light), the stranger, Goodman's name itself (good man)- There are many many more symbols, however, since Hawthorne is heavy on them in all his works.