The overall quality score consists of calculations and algorithms that determine the value of an influencer and is optimized on how successful collaboration could be. If you can expect the natural reach and engagement, you would expect an influencer similar to the size. 
Good performing influencers should have a value of at least 8, and average performing influencers should have a value of 5 and more. 
This score results from the Mini Scores, which all put the influencer in comparison with influencers that have a similar size.