https://marco.org/2008/06/20/from-tumblr-staff-testing-dashboard
From Tumblr Staff:
Testing: Dashboard Notifications
We’re trying out a new feature and are really curious to hear what you think.
Notifications appear on your Dashboard for things like new followers and reblogs. It still has some glaring holes, but we’ve been loving this feature for discovering new people on Tumblr.
You can enable Notifications on your Preferences page.