It is to sing the entire piece through in your head.
The purpose of this training is to develop a skill to play through even when you hit the wrong note or get bowing wrong. At the time of practice, it is very common to stop playing every time you make a mistake, focus on the part you made a mistake in, and then go on to play the rest. But if you keep practicing like this, you are likely to develop a habit of stopping, and you will get distracted by many things at the time of your recital. To prevent a situation like this, you need to practice to not stop no matter what happens and play through. Trying this at a different time of the day is very effective, such as the end and/or beginning of your daily practice, or at the exact same time frame as your scheduled performance, etc.
Another important tip is to never get down or start wondering why you made a mistake during a full rehearsal. What’s important is how quickly you recover from your mistakes. Don’t ruin the rest of your performance by dwelling on the errors. This is one of the essential points in mental practice.
To make full rehearsals even more effective, have someone listen to your performance or record yourself. Training yourself to be able to sing the entire piece in your head anytime anywhere is also very important.