Juice fasting is when you only drink juice and water for a few days or even weeks. People do this to give their body a break from solid food and help their body get rid of any bad things in it.
Just like when you get sick and your body needs rest, when you go on a juice fast, your body gets to rest and use all the energy it would normally spend on digesting food to heal and get rid of bad things.
But, it's important to remember that juice fasting is only safe for a short amount of time and you should always talk to a grown-up before trying it.