Okay kiddo, have you ever played with a toy xylophone before? Imagine you have one with ten keys on it. When you hit the first key, it makes a sound that we call a "note". When you hit the second key, it makes a different note that is higher or lower than the first note.
Now, imagine you had someone help you tune your toy xylophone so that all the notes sounded really good together. This is called "temperament". There are many different ways you can tune your xylophone, just like there are many different ways musicians can tune their instruments.
One way musicians used to tune their instruments was called "meantone temperament". This means they set the notes on their instrument so that certain ones sounded really nice and in tune, while other notes might sound a little bit off.
The way they did this was by making certain notes slightly higher or lower than what we call "pure" tuning. It's kind of like you tuning your toy xylophone so that some notes were closer together and some notes were further apart.
Meantone temperament was used a lot during what we call the "Renaissance" period of music, which was hundreds of years ago. Nowadays, most musicians use a different kind of temperament called "equal temperament", which makes all the notes sound equally good together.