The Ruzsa-Szemerédi problem is a very tricky question that asks if we can find a pattern in sets of numbers, even if we don't know much about what the numbers are.
Imagine you have a big bag of balls, each with a number on it. You can dump out the balls and put them into different groups. But you don't know anything about the numbers on the balls - they could be any combination of numbers!
Now, let's say you want to find out if there's a way to group the balls so that the difference between any two numbers in a group is the same. For example, if you grouped together balls that had numbers 1, 3, and 5, then the difference between 3 and 1 is 2, and the difference between 5 and 3 is 2 as well.
The Ruzsa-Szemerédi problem asks if there's always a way to find these kinds of patterns in a set of numbers, no matter what the numbers are. It turns out that this is a really hard question to answer, and mathematicians have been trying to figure it out for a long time!
In fact, there are still many things we don't know about the Ruzsa-Szemerédi problem. But as mathematicians keep working on it, we'll learn more and more about how to find these kinds of patterns in sets of numbers.