In "Fahrenheit 451" what is the symbolical meaning of the following description of Clarisse's house?"The house that was so brightly lit this late...

In Montag's society, everyone is living in intellectual darkness, and in blind distraction from things that matter.  Mildred, Montag's wife, is so miserable under the surface that she tries to end her life; during the day, she drowns out her sorrows with over-entertaining herself.  She gets nervous without her seashells in her ears distracting her from her thoughts, or without the t.v. walls constantly pulling her into meaningless and empty "relationships" to keep her busy.  Everyone in their society does not talk to one another in meaningful ways anymore; instead, they just go smash things up or plug into entertainment or noise.

The light of people's minds in Montag's society has gone out.  At night, no one has the lights on, because they are all sitting around the t.v. walls, staring, silent, not interacting.  Clarisse's house is bright--her family is talking, they are connecting, forming real and meaningful relationships.  And, they are one of the few left in their society, so, they stand out.  The lights on in their house symbolize the light that is in their eyes and minds; intelligence, connecting, questioning, enjoying life and all of the small things.  Pretty soon however, that light is noticed, and is snuffed out, just like any person that stands out and attempts to search out happiness in ways not encouraged by their society is not-so-gently reminded to conform, or else.  Their society snuffs out the bright ones who question things early.  Beatty states,

"You can't rid yourself of all the odd ducks in just a few years.  The  home environment can undo a lot you try to do at school.  That's why we've lowered the kindergarten age year after year until now we're almost snatching them from the cradle...Luckily, queer ones like [Clarisse] don't happen often.  We know how to nip most of them in the bud, early."

If they can't "nip them in the bud" while they are young, they send the firefighters to be a bit more persuasive.  So, one by one, person by person, house by house, anyone who has the light of intelligence, independence and thinking left, is darkened.  And, their entire society, in the dark, is left to destroy itself.

I hope that those thoughts help to point out the symbolism a bit; good luck!