It's a thing Tumblr is trying out, to get people with Tumblr accounts to put a portion of their content behind a paywall... From which, Tumblr will take a cut (naturally).

The really bad thing is that, in Tumblr's first announcements of this shiny "new feature," they were encouraging account holders to get paid for their fan art and fanfic... which is illegal. And also, in the fine print about Post+, they absolved themselves of all responsibility, and won't defend any user if they get sued for their fan art or fanfic.