Pick any number. If that number is even, divide it by 2. If it’s odd, multiply it by 3 and add 1. Now repeat the process with your new number. If you keep going, you’ll eventually end up at 1. Every time.
Mathematicians have tried millions of numbers and they’ve never found a single one that didn’t end up at 1 eventually. The thing is, they’ve never been able to prove that there isn’t a special number out there that never leads to 1. It’s possible that there’s some really big number that goes to infinity instead, or maybe a number that gets stuck in a loop and never reaches 1. But no one has ever been able to prove that for certain.