Compound Interest is not a difficult concept to understand. However, it is a concept that not many adults seem to understand let alone know about.
In a nutshell, compound interest is about putting your money into something and earning interest and re-investing that interest. So over time the initial amount (the principal) and the continuous interest add up to form a substantial amount of money. If you keep on adding to that same investment as well, then the result is even more dramatic.
It is interesting to see what happens when time and compound interest start to happen. There are loads of compound interest calculators online. Here are a couple
This is a great short video on Investopedia explaining compound interest.