Tonnetz is a way to show how different musical notes are related to each other. Imagine you have a lot of different colors of crayons. You can line them up in a row from light colors to dark colors, and you can put colors that are similar next to each other.
In music, we can do something similar. We can take all the notes that make up a big musical scale and line them up in a row. From there, we can start putting notes that sound good together next to each other. We can also put notes that are further apart from each other when they don't sound as good together.
The tonnetz is a special way to show which notes sound good together and which don't. It organizes the notes into a grid where notes that are close to each other on the grid sound good together. Kind of like how colors on the crayon box that are close to each other look good together in a drawing.
Musicians can use the tonnetz to help them write new music. They can look at how the notes on the grid are arranged and use that to decide which notes to play next. So just like when you use your crayons to make a beautiful drawing, musicians can use the tonnetz to create beautiful music!