The Followers insight gives you a trending view of the total number of followers to your Pinterest account. (This number includes followers to particular boards, as well.) In the given time period you can see the followers to your account and whether the follower count is trending up or down each day.
Followers in Pinterest is the reach your Pins have. The Pins you share will show up on followers' feeds for them to engage with you. The more followers you have, the more audience you have to market and sell to. If you see an increase in your follower count week over week, take a look at the content you are sharing and check out the Engagement and Engagement Rate by Board insights to see which boards are most active. Keep sharing content that is popular with your audience.