Hi Kani_B,
Unique Trait Realization: The total amount of unique users that qualify for the trait in the time range.
Example: A user that would visit your homepage three times on the 1st of July would count as 1 user/count in this metric.
Total Trait Population: The total of amount of users currently qualified for this trait. This is meant to help show total amount of users who could be used for segmentation/targeting. Typically, users will remain part of a trait for 120 days.
Example: A user that would visit your homepage three times on the 1st of July would remain 1 user part of this population every day until October 29. On October, 30, the user would be removed from this population.
Thanks,
Shweta