Content listed under these tags are
chosen by the editors. These editors are users who are chosen by a
semi-automated process. Tumblr’s bot identifies users who post popular
content about a topic. Users from this list are then selected to be
editors. Every 1-4 weeks, editors are switched out.

Their FAQ answers all of these
questions: tumblr.com/docs/en/editor_faq

• The
  Top Editor in each tag is the person whose featured posts get
  the best response from the Tumblr community.

PS:  agreed with @伍超， add some details.