In fiction, theme is what the story is about. So a romance novel will be, on the surface, a story about two characters falling for each other. But looking deeper, the themes will be what the work of fiction has to say about the fictional universe which the story is set in, and to our universe. Love is worth the pain that precedes it; love is where you least expect it; true love conquers all, and so on.
For Jurassic Park the most obvious theme is that dinosaurs are cool, but few films can become as successful as Jurassic Park was if that’s all they have to say. There’s also a key theme that it’s dangerous for humans to think that they can control their new technology – Frankenstein ‘playing god’ theme. It’s a recurring theme in Michael Crichton’s fiction, including Westworld and Prey. Another Jurassic Park theme is the importance of family, in this case the non-traditional ‘family’ that forms between the central characters in the process of protecting each other.