Thunderbird question?

Does anyone know what the little icon that looks a bit like DO NOT ENTER sign that has suddenly appeared on the left end of some of the message lines in my Thunderbird inbox mean?

(Okay, not just "suddenly appeared": I have the bad habit of quickly typing a few random keys when my screen blanks unexpectedly. Which is what I did when it went black about 20 minutes ago. When my screen lit up again, a bunch of messages (apparently everything in one thread) had acquired this icon. Which neither my Google-fu nor cruising the menus have helped me figure out so far.)