The Overall Quality Score is a number in the range from 0 to 10.
It is a weighted average of the individual scores. The weight of each individual score is decided by the quality of the impressions. The quality of an impression is determined by how many quality metrics it violates.
For example, if your campaign has a bad Viewability score and a bad Geo Compliance score, but the impressions causing these to be bad are not the same, then both scores will be given less weight when computing the average. On the other hand, if both scores are bad and it is caused by the same impressions (being both out of the country and not viewable), then they'll be given a higher weight in the average. The second example will, therefore, give a lower overall Quality Score because the quality of the impressions of your campaign is lower than in the first example.