Penetration is calculated always for the audience you are analyzing on a specific psychographic trait or interest and it tells you what percentage of the people who have a psychographic trait/interest also fall in the audience you are analyzing. An example will make understanding easier:

Analyzed Audience: Outdoor Voices

Interest: Top Products

In this example Penetration tells us that 2.29% of Glossier’s audience is also interested in Outdoor Voices, that is our target.

Tip: use Penetration index when you are evaluating potential spokespersons and partnerships with other brands to pick the celebrities/brands whose audience overlaps the most with the target you want to reach.