For a long time, it was common for a young person seeking to learn a trade to apprentice with an older, experienced person. This apprenticeship served to pass on useful skills and develop real-world experience in working. Manolin is an apprentice fisherman in a fishing village; many of the men in the village work on the water, and so it is normal for Manolin to apprentice with one fisherman or another. He sails with Santiago for forty days, during a long streak of bad fishing:
But after forty days without a fish the boy's parents had told him that the old man was now definitely and finally salao, which is the worst form of unlucky and the boy had gone at their orders in another boat which caught three good fish the first week.
(Hemingway, The Old Man and the Sea, Google Books)
This, again, is typical; if Santiago is not catching fish, switch to a fisherman who does catch them. However, during their time together, Manolin becomes enamored with Santiago's personality and his experiences, and they become good friends with a father-son relationship. Manolin is the innocent side of Santiago's personality, sharing many of his interests but looking on them with fresh eyes; Santiago, for his part, sees Manolin as someone who should aspire to greater things than Santiago did in his own life, but also enjoys the boy's attention and loves him as a son. By the end of the story, Manolin has resolved to learn from Santiago no matter what his parents say, thus ensuring that Santiago's life and experience will live on.