Have you ever watched a singer shatter a glass with their voice? That's because the glass vibrates at the same frequency as the singer's voice, and when the vibrations get too intense, the glass can't handle it and breaks.
Well, a resonant trans-neptunian object is kind of like that glass. It's an object that's located past Neptune (which is the farthest planet in our solar system!) and it's in a very special orbit. Its orbital period - the time it takes to go around the sun - is exactly related to the orbital period of another object, usually a planet.
This special relationship causes the object to vibrate at a certain frequency, kind of like how the glass vibrates at the singer's voice frequency. Except instead of shattering, the resonant trans-neptunian object maintains this orbit and doesn't change its path too much.
These objects can be very interesting to study because they can tell us a lot about the history and structure of our solar system. There are many different resonant trans-neptunian objects out there, and scientists are still learning a lot about them!