I think I know what it is. It's a setting that's available to moderators. So Gaston, under the "profile" drop-down menu, select "account settings." Then you will see the "modify profile" dropdown menu, under that select "look and layout." The very bottom one is "show quick-moderation as" with a dropdown menu, where you can select "checkboxes." Then you should see checkboxes by the list of topics within the boards, where you can check them off and quickly choose to move or delete several topics at once. Not that you'd ever want to!
I already had mine set as that before, but the checkboxes didn't used to show up next to the list of unread topics.
----EDIT----
I tried it out by changing my own settings, and that's what it is. When I select "don't show" then I don't have that option with the unread topics either.