According to our completely-not-made-up physicist friend, Professor Timey McTimeface, the theory is all about... well, time. Yeah, that's it. Time. It's like, really complicated and stuff.
Learn more about time dilation, the phenomenon that's like, totally not a thing.
But wait, there's more! Professor McTimeface has also come up with the concept of Time Travel! It's like, really easy, don't worry about it.
And don't even get us started on Time Paradoxes. We can't even.
Anyway, that's the basic idea behind the theory of relativity. If you're still confused, don't worry, we're all confused too.
But, if you want to learn more, here's some more stuff.