The marlin can have several symbolic meanings depending on how the story is read. At the simplest level, the marlin is an enormous fish that Santiago catches through determination. Going deeper, the marlin represents an ultimate, a perfect goal that comes once-in-a-lifetime, and so for Santiago catching it represents the culmination of his entire life.
I want to see him, he thought, and to touch and to feel him. He is my fortune, he thought.
(Hemingway, The Old Man and the Sea, Google Books)
Another interpretation of the marlin is that it is Santiago's reclamation of his reputation; he has had hard luck recently and some of the other fisherman make fun of him, but he remains confident that his skills have remained strong and his luck will change. Another interpretation is that the marlin is the final obstacle in a man's life. It proves almost too strong for Santiago, but he defeats it with willpower; had he been weaker, the marlin would have killed him with its strength.