Hi Esraa, “to fall off the wagon” means to fail in maintaining a new habit you were trying to establish. Like breaking the new habit. As said in other comments, it’s related to addiction. For example, someone is an alcoholic, but they stopped drinking (alcohol) for a year. If they have a drink at that time, we say they fell off the wagon.

It means to stop catching up with a plan or schedule😊

Falling off the wagon is often used in the case of people who are trying to give up drinking alcohol, or a bad habit. When they 'fall off the wagon', it means that they are no longer sticking to their goals as diligently as they should be, or they have given up altogether.