In Joseph Conrad's Heart of Darkness, the rivets are an important element necessary in Marlow's ability to move forward on his mission to bring Kurtz back—if only he can repair his steamboat, which has been run aground at the second station.
Usually symbols support the themes of a story. In this case, I believe that the theme that the rivets support is that of order and disorder. Marlow has seen a great deal of this since his arrival in the Congo, working for the Company. At the first station he sees the graveyard of machinery rusting to nothing, the dynamiting of the cliff for no good reason, and he recalls boxes of rivets lying about, unused—useless. The rivets should be such an easy thing to come by because there are so many abandoned in large numbers, ignored.
Symbolically, I believe the rivets represent Marlow's inability to achieve his purpose, as well as how he handles the delay. It is during this time of inactivity, that Marlow has time for meditation—studying his situation, and reflecting upon the person of the mysterious Kurtz; how may life in this jungle have affected Kurtz?—perhaps even Marlow sees the Congo affecting him. Marlow experiences frustration over the rivets, but also becomes aware of the disorder of life within the jungle, which affects the people there.
I had given up worrying myself about the rivets...I said Hang!— and let things slide. I had plenty of time for meditation, and now and then I would give some thought to Kurtz. I wasn't very interested in him. No. Still, I was curious to see whether this man, who had come out equipped with moral ideas of some sort, would climb to the top after all and how he would set about his work when there.
Marlow could well be speaking of himself in this quote—there are still many harrowing experiences ahead of him. The rivets prevent Marlow from repairing the ship, but their failure to appear allows Marlow the opportunity to use his time to contemplate the nature of the jungle and its effects upon the men who enter there.