Jazz is a kind of music that started a long time ago. In the 70s, jazz music changed in some ways.
First, some musicians started to play jazz that sounded a little bit more like rock music. They used electric guitars and drums that were a little bit louder. This was called "fusion" jazz.
Second, some musicians started to play jazz that was more free. That means they didn't stick to any plan, they just played whatever came to their minds. This was called "free" jazz.
Lastly, some musicians played a kind of jazz that sounded a little bit sad or slow. This was called "cool" jazz.
So in the 70s, jazz changed a lot. Some jazz was more like rock music, some jazz was more free, and some jazz was more cool and slow. It was a very interesting time for jazz music.