Reach is the main percentage on the target and it tells you what percentage of people in your audience (in the example Outdoor Voices) has a specific psychographic trait, demographic characteristic or is interested in a specific brand/media/influencer.

Let's make an example to understand the concept better:

Target: Outdoor Voices

Interest: Top Beauty Brands

Reach tells us that 9.46% of those interested in Outdoor Voices are also interested in Glossier, or also that 9.46% of our audience (Outdoor Voices) is interested in Glossier.