First of all, let's start with the basics. What are neutrinos? Neutrinos are tiny, almost massless particles that are everywhere around us. They are a bit like ghost particles because they hardly interact with anything. Every second, trillions of them are flying through your body without you ever noticing. Neutrinos are produced in nuclear reactions like those in the sun, nuclear reactors, and, as you've probably guessed, during supernovae.

Now, onto the fascinating part—neutrino oscillation. This is where things get interesting and a little weird. Neutrinos come in three types or "flavors": electron-neutrino, muon-neutrino, and tau-neutrino. Imagine if you had three different flavors of ice cream: vanilla, strawberry, and chocolate. Now, imagine you're holding a scoop of vanilla ice cream, but as you're walking, without you doing anything, it magically changes into strawberry and then into chocolate and back. That's kind of like what happens with neutrino oscillation! A neutrino that starts as one flavor can change into the other flavors as it travels.

Why does this happen? Well, it all boils down to a bit of fascinating quantum mechanics and the mass of neutrinos. Initially, it was thought that neutrinos had no mass. However, the phenomenon of oscillation provided evidence that neutrinos actually have a very tiny mass. This mass allows them to shift between their different states or flavors.

Let's explain this further with the concept of superposition. In quantum mechanics, particles like neutrinos can exist in a combination of states until they are observed. This means a neutrino isn't strictly just one type of neutrino till it's detected. It's a combination—a superposition—of all three flavors. As it travels, the proportions of these flavors change, leading to the oscillation we observe.

You may wonder how we know this happens when neutrinos are so hard to detect. Scientists have set up massive experiments, sometimes deep underground, using special detectors filled with ultra-pure water or another substance. When a neutrino interacts with these detectors, it creates a tiny flash of light. By observing these interactions over time, scientists have noticed the shifts from one type of neutrino to another.

So why is neutrino oscillation such a big deal? For one, it challenges the Standard Model of particle physics, which initially predicted neutrinos to be massless. By proving that neutrinos oscillate and hence have mass, scientists have had to rethink some of the fundamentals of how particles work.

Moreover, understanding neutrino oscillation better could have significant implications. It might one day help explain why there is more matter than antimatter in the universe, a major question in cosmology. Plus, it deepens our insight into the secrets of the universe, the forces that govern it, and the behavior of some of its most elusive inhabitants.

In conclusion, neutrinos are more than just ghostlike particles zipping through space; they are fundamental in piecing together the puzzle of the universe. Neutrino oscillation reveals the hidden complexity of these particles and provides crucial hints about the fabric of reality.

That's all for today's journey into the world of neutrinos and their oscillating dance. Thanks for tuning into ELI5. I hope this was informative and maybe even a bit entertaining. Remember, the universe is a big place with lots of mysteries, and we're here to explore it together, one question at a time. Until next time, keep questioning and stay curious!