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: MorningStar Farms
Interest: Top Chefs
In this example Penetration tells us that 2.63% of Justin Warner’s audience is also interest in MorningStar Farms. This information tells us that he would be a good spokesperson for MorningStar because a high percentage of his audience is interested in the brand (compared to other chefs).
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.
What can be defined as a good Penetration?
Penetration changes a lot from category to category so the best thing is to look at the percentage in the specific category you are analyzing, get an idea of the average penetration and see where the penetration is above the average penetration of the category.