Documentation is basically a fancy word for writing down important information about something or someone.
Imagine you have a new toy that you love to play with. You want to keep it safe and take care of it, so you ask your mom to write down how to clean the toy, what it is made of, and how to fix it if it breaks.
This is what documentation is - it's writing down important information so that you or someone else can remember how to do something in the future. Some common examples of documentation for grown-ups include instruction manuals for electronics, medical records, and job descriptions.
Just like with your toy, having good documentation helps keep things safe, organized, and easy to understand. So, whenever you want to remember how to do something or explain it to someone else in the future, you can look back on this documentation and know exactly what to do.