The Traffic % (shown in the Organic Positions & Advertising Positions reports) coincides with the overall traffic number shown in the top left on both reports. You will notice a Traffic % for each keyword the queried domain ranks for. This represents how much of the overall traffic is coming from the specific keyword.
For example, if I search buzzfeed.com and want to understand how much traffic the keyword “knock knock jokes” is bringing to this domain, I can look under the Traffic % column. As you can see in the screenshot, “knock knock jokes” has a Traffic % of 0.37%. This means that 0.37% of the overall 31 million average visitors is coming specifically from the keyword “knock knock jokes”.