High-energy physics is when really smart people study really tiny things like atoms, protons, and neutrons. They use big machines called accelerators to make these tiny things move really, really fast. Then they smash them together and watch what happens.
By smashing these tiny things together, they can learn a lot about what the world is made of and how it works. They can also find out how different particles interact with each other and how they're related.
It's like playing with tiny building blocks and seeing how they fit together to create bigger and more complicated things. Scientists use high-energy physics to understand the basic building blocks of the universe and how they work together to make everything we see and know.