Inactive Users

I searched on the forum, but some posts turn up in the results and don't seem to exist (type "membership inactive").

My question is this, what are inactive users? I get a lot of spam users with this plugin as it circumvents the email verification (I don't get why, this is such a common practice!). However, it seems like there is some sort of anti-spam going on, as so many users are inactive. What does that mean?