Moby Dick’s symbolic meaning is hard to pin down, and the indeterminateness of it is in large part the point of Melville’s novel. There are a few things we can say for sure:

1. Moby Dick is, firstly, the object of Ahab’s vengeance. The whale bit off his leg, and in so doing represents for Ahab the essential hostility of nature towards man.
2. Alternatively, Moby Dick represents the blankness or indifference of nature towards man. That is, while Ahab may seek vengeance on the whale, the whale itself doesn’t know anything about Ahab.
3. The whale can be seen as an avatar of God or some divine power. In this case, Ahab’s revenge is more cosmic in scope; his quarrel is with God, not the whale.
4. The whale can be seen as symbolic of a certain lack of meaning – however much we want to assign significance to reality, the whale symbolizes how “inscrutable” reality truly is.

And so forth. Some key chapters to look at where this idea of symbolism is developed are “The Whiteness of the Whale,” Chapter 42, or “The Quarterdeck,” Chapter 36. Good luck!