Envelops?

[Sigsog226]

I've been looking around on BF for the answer but I can't find out what the envelops mean at the left side of the thread listings. Some are gold, gray, blue, and then some have multiple ones on them, some open closed. I'd appreciate if someone could break it down for me. I'm sure I'm going to feel stupid once I get the answer. Thanks

 

[EyeDog]

When in a forum look at the bottom of the page for a list of ICONS.

 

[Sigsog226]

I've been using this BF for about four years now and never noticed it. Thank you!