The formula of the traffic estimate is as follows:
Search volume * click rate of the site's position on the keyword
My site is 1st on the insurance keyword which has a search volume of 10,000 (10,000 users type this keyword each month).
The click rate of the 1st position is 37%.
So, the traffic estimate could be 10,000 * 37 / 100 = 3700 visits per month
Note that it is possible to enter your own click rates on the tool to calculate a traffic estimate. It is then necessary to access the "project edition":
Scroll to step 3:
Then click on activate. You can specify different click rates based on brand or non-brand keywords (provided you have tagged the brand keywords as is).