That is an enormous question - one that cannot be answered in 1500 characters! Look at the Salem on Literature link for specific details on what you are asking about.
This is a beautifully written ugly play (by one of my favorite playwrights-Lanford Wilson). It has many of the hallmarks of his writing: complicated plot lines that weave together seamlessly, multi-dimensional characters that slowly are revealed to the audience during the course of the play, and poetic language and beautiful literary devices (even in the midst of ugliness).
The themes, while about the seamy side of small-town American life, are ones that may apply more universally. They are certainly things we should consider in our own lives, no matter where and when we live -- hypocrisy, living in the past, surface values vs. practiced values.
The main symbol is that of the rime - the frost that is beautiful to look at, making even ugly things beautiful. This is part and parcel of Wilson's point about the superficiality of the town's stated morality and the ugliness of their reality as evidenced by their behavior.