Ad Impressions is the count of how many times an ad loaded / served. The Ad CPM is the average CPM of those ad impressions.
Unit Impressions is the count of how many times an ad slot loaded on the page and served at least 1 ad impression; refreshes are not counted as additional impressions. The Unit CPM is the average amount earned per each unit impression, so more refreshes will result in an incrementally higher Unit CPM.
Ad Impressions/Unit Impressions = avg number of refreshes.