The island is a rather remarkable and fantastical element to Yann Martel's "Life of Pi," and requires the reader to really suspend their disbelief in order to believe that it could actually exist. In interviews, Martel himself said that he put the island into the story in order to require the reader to take a leap of faith. The entire story thus far is pretty interesting, and takes a lot of faith to believe in the first place. Pi, surviving for weeks on end in a life boat with a carniverous and vicious tiger? Martel makes it pretty believable, through his use of Pi's expertise in zoology though, so, we can flow with it. But, an island filled with lemur-like creatures and acidic and deathly tube plants, that is just floating out in the middle of the ocean? Now that takes a huge leap of belief to swallow.
Since Martel's theme in this book was the power of storytelling, and how a really great story is like taking a leap of faith, he wanted to make the readers leave behind any rational logic and leap into the unknown, and believe something just because someone said it existed, and because it was a great story. The island symbolizes faith, and how when you have faith, you take a leap in the dark, and believe in things that might seem impossible to other people. That is what the island is--something that might seem impossible, but who's really to say it doesn't exist? Martel wanted the reader to experience that leap of faith, believing in a really great story at the behest of logic, and how it can be invigorating, interesting, and comforting. His book is all about "the power of the imagination," and the island definitely is a part of that. I hope that helps; good luck!