To get 'Organic Traffic' numbers, we put every keyword into Google and check the position where your site ranks. That’s where we get positions from (more details here).
Then we take estimated monthly traffic from Google’s records, apply for the tracked position and calculate how much traffic (approximately) this keyword should bring.
Thus, the traffic roughly equals search volume multiplied by the percentage of clicks for a given search position.
So if the first position receives 30% of all traffic, and the keyword is searched for 20 times a month, the estimated traffic would be 20 * 0.3 = 6.
It is worth mentioning that traffic numbers reported by Ahrefs will differ from those reported by Google Analytics.