User freshness is the amount of time since the user signed up to receive push notifications from the website. It tells how long the users have been receiving push notifications and thus how much "banner blindness" they may have.  

We have found user freshness to be the single greatest factor affecting conversion rates and CPA.  Users with a user freshness less than 7 days have seen significantly higher ROI. The downside, however, is that selecting the freshest users will greatly restrict your audience size and thus your ability to scale. By selecting only the first couple hours, you could be restricting your ads to only 1% of our total traffic.  We encourage you to test with a fresher audience group and then scale to the rest of the audience freshness groups.

Not all of our supplies provide use with the time and date of when the user first allowed push notification, so we don't know the user freshness of this group.  It could be 1 hour or 30 days.  Consider this group an average.