A symbol is an object or person that stands for something other than its literal meaning. An object can be a symbol, such as the books that Montag burns. A person can also be a symbol.
I will explain how Clarisse is a symbol of change. Since she is young and different from the others. Her family has been in trouble for not conforming to the fact-paced society. She confronts Montag with some hard truths. Even though he sees her only briefly, she stirs something in him.
Of course I'm happy. What does she think? I'm not? he asked the quiet rooms. (part 1)
Clarisse is actually present in the book very briefly, because she gets killed. This is also part of her job as a symbol. She is a fleeting glimpse of another self, or another world, for Montag. He has to choose whether or not to follow her lead and actually change. He chooses change, and his life is never the same after that. He chooses to question what is in the books, and what a life with books might be like.