The novel is partly based on real events that happened in 1820 with the whaling ship "Essex" (in the novel it is called "Pekvod"). The work contains a lot of symbolism and subtext.
A white whale nicknamed Moby Dick destroyed more than a dozen ships and sailors. And it was he who bit off Captain Ahab's leg a few years ago. Ahab wants revenge on the animal and he does not care how and at what price.