Okay kiddo, imagine you have a really fast spinning toy top. When you spin it, the top flattens out and you see a flat disk shape instead of a round one.
Now imagine this toy top is going incredibly fast, like really really fast, almost as fast as light. When it spins that fast, some crazy things start happening. The disk shape starts to look different from different angles, almost like it's squished or stretched out. This is because of something called the "relativistic effect."
This effect happens when things move really fast, especially close to the speed of light. When you move that fast, time and space start to work a bit differently than we're used to. This can cause objects to look distorted or "warped," and that's what's happening with our spinning top.
In space, there are things called accretion disks which are made up of gas and dust swirling around a black hole or other massive object. These disks can spin really fast, and because they're moving so quickly, they can also look distorted or "relativistic." This can help scientists understand more about how black holes and other cosmic objects work.
So to sum it up - a relativistic disk is a disk of gas and dust that's spinning so fast, it looks distorted or "warped" because of the way things work when you move close to the speed of light. It's like a super-fast spinning toy top in space!