Symbolism, Imagery, Allegory
In The Hitchhiker's Guide to the Galaxy, The Heart of Gold is the spaceship that Zaphod steals, including the engine that drives the spaceship—also known as the Infinite Improbability Drive. It's crucial to the plot since it allows Zaphod and crew to find Magrathea. It also sends some police after Zaphod. But that's not why we're including it here as a symbol.
We're including the Heart of Gold mostly because of its name: a "heart of gold" usually means someone who is nice. As in: "Zaphod Beeblebrox has a heart of gold—he would pick up hitchhikers even when he's running from the police." That would mean that Zaphod is an all-around nice guy who genuinely cares about other people.
But that sample sentence is pretty much totally wrong in this book. Zaphod does have the Heart of Gold starship, but he really doesn't have a heart of gold. When the ship rescues Arthur and Ford, Zaphod half-heartedly approves of the rescue. He says he wouldn't have been happy if the hitchhikers had died, "Not as such, but…" (11.11). Friends, let's call a spade a spade. "Not as such, but…" means: "I totally wouldn't mind if other people died if the other option is me being inconvenienced." Zaphod doesn't have a heart of gold at all.
Now, sometimes we use "heart of gold" to indicate that someone is nice even though we don't expect them to be nice. For instance, we could say "that disgusting Prostetnic Vogon Jeltz has a secret heart of gold—just listen to his poetry and you'll learn what a nice guy he secretly is." But that's totally wrong here also. When Arthur and Ford lie and tell him that he writes poetry to express his inner heart of gold, Jeltz corrects them. He doesn't write poetry to show his love: "I just write poetry to throw my mean callous heartless exterior into sharp relief" (7.35). In other words, he's got a mean exterior that hides… a mean interior. His heart of gold is totally fake.
So "Heart of Gold" is a funny name for a ship that can travel all over the galaxy, especially since the galaxy seems pretty lacking in truly kind-hearted people. It sure is pretty full of people pretending to be all kinds of things for their own benefit, though.