Not Provided Sessions is a proprietary metric that estimates the number of organic sessions drived by each keyword, based on Clicks from Google Search Console and Google Analytics Sessions.
The amount of not provided sessions reported by Google Analytics is divided and assigned to keywords, proportional with their number of Clicks from GSC. However, keep in mind that the number of GSC Clicks will rarely match the number of GA Sessions for a webpage.
Also, while we'll be able to estimate the source-keyword for most part of your not provided sessions, there will often remain a small number of unallocated not provided visits for a website.
Do you have any other questions? Don’t hesitate to get in touch and we will keep building the FAQ.