I can explain it with an example.
Let us say you are able to see 10 traits on the left panel in the Trait to Trait overlap report, and you selected date as last week's date and selected 7 day range.
Now, if you select a base trait (123) from the left panel, that means you will be able to see trait to trait overlap report for all traits against this base trait.
Now if for the first trait in the list for example : trait id : 456, you see Overlap % : 20%, Base Trait Uniques : 3.4 Million, Overlapping Trait Uniques : 1.4 Million
This means 7 day uniques of Overlapping trait is 1.4 Million, and the 7 day uniques of Base Trait is 3.4 Million. You can also see these numbers if you see only General Trait report or Trait graph for these trait IDs.
Now, the 20% overlap means that base trait's 20% 7-day unique population can be found in overlapping trait's 7-day unique population. So if we calculate the 20% of 3.4 million it would be : 680k. So 680k Device IDs in the overlapping trait id: 456, are also present in base trait : 123. You will then see overlap uniques as 680k.