Imagine you have a toy box with a certain number of toys inside it. You pick up one toy and hold it in your hand, while the rest stay inside the box. After a while, you put the toy back inside the box and continue playing with the other toys.
The Poincaré recurrence theorem is a mathematical concept that explains what happens if you keep repeating this process over and over again, for an infinite amount of time.
Basically, what the theorem says is that if you keep taking out one toy and putting it back into the box, eventually – and I mean REALLY eventually, like after a ridiculously long amount of time – all the toys will be back in the box in exactly the same way they were at the beginning. This is known as a recurrence event, and it is guaranteed to happen eventually if you keep repeating the process.
Now, this might sound crazy or impossible, but it actually applies to a lot of different things in the real world, not just toys in a box. For example, it also works for planets in the solar system, or particles in a gas.
The Poincaré recurrence theorem is pretty advanced math, but the basic idea is that everything in the universe is interconnected in complex and unpredictable ways, and given enough time, these connections will eventually cause things to repeat themselves. So, while you might not live long enough to see it happen, don't be surprised if the entire universe eventually ends up exactly where it started!