Doctors use it to work out how much pain you're in. You can use it too - it's called a scaling question.
Scaling questions may sound a bit odd the first time you use them. You might need to hold your nerve and keep quiet after you've asked one just so they think things through and give you an honest answer.
So how do they work? Imagine you've just asked someone if they agree with your proposal. They say "yes" but you're not convinced they mean it. Just ask:
"On a scale of on to five how much do you agree?"
and wait for the number they give you.
Doctors ask this kind of question when their patient is in pain and they want to understand how bad it is. There is no easy way for the doctor to know the pain level. All they have to go on is the patient's own assessment. In gyms up and down the country personal trainers get their clients to work at "level 7" then push it up to "level 9". These measures are subjective but useful. In fact doctors refer to them as subjective units of distress.
So how does this help you? Well when they answer your question and say their agreement is about a "three" you know you still have work to do.
The trick is not to focus on getting a high score to just get them to give you a number. Once you have score you have something to work with.
A great next question could be:
"What would make it a four?"
In other words just a movement of one point. Surprisingly people generally know what's needed to make an incremental improvement.
The scale can be bigger than five. Try one to ten or even a hundred. The key is to get this shared frame of reference and for them to give you a number.
You'll find you understand them better and they will trust you more.