Could someone explain the envelopes?

[Mickey1122]

Next to every thread, I see envelopes, some blue and some read. Some have arrows. What does each mean? I can't seem to find an explanation for it anywhere.
Thank you in advance!

 

[Sparx]

there is a key down at the bottom of the front page of every forum.

 

[Mickey1122]

Thank you. I can't believe that I didn't notice that before.

 

[HSMndLKfan92]

Thank you. I can't believe that I didn't notice that before.

That was the first thing I noticed once I joined. Anyway, back to life.