In this great novel, Ahab is obsessed with hunting down the great white whale. It consumes him. The cat-and-mouse game, of sorts, can represent the attempt by people to understand God and his will, which is simply not possible. No one can truly know what God will do or why he did or does what he does in this world. Obviously, this is the way God wanted it. This is also true of nature and its creatures, including the whale.
For much of the novel, the whale is hidden from plain view, so we do not get a good idea of its entire physical appearance. This can also represent the fact that God is also never fully revealed to us as humans. We trust in Him and know He exists (if we are Christians, for example), but He is never truly revealed to us.