A "boffin" is a term that grown-ups sometimes use to describe someone who is really good at something complicated or difficult, usually related to science or technology. It's like calling someone a "smartypants" or a "brainiac."
Think of it like this: you know how some kids are awesome at playing video games, or maybe really good at reading? Well, boffins are like those kids, but they're grown-ups who are really good at things like math, science, or figuring out how to make cool things work. They might spend a lot of time in a laboratory or at a computer, working on experiments or projects that most people don't really understand.
Boffins are the ones who make new discoveries or come up with new ideas that help us learn more about the world we live in. They're kind of like superheroes, except instead of saving the world from bad guys, they help make the world a better, more interesting place by using their brains to solve really hard problems.