this means you are using the admin bar plugin and having it gather unread posts…. but you are not clearing those posts from the admin post bag…  the warning is just telling you its getting large and affecting your site performance…   if you are not using the admin bar, you should deactivate it…

I’m also getting it.  Where do you go to clean it out?

From within the Admin bar there is a link/icon to empty the Admin postbag.