What does pinned mean and what is the significance of the colours of the envelopes next to each topic title?

The colours all relate to new topics, popular new topics and whether thay have been read yet.
If you look at the bottom of the page when you are about to post it should give you a description.

Hope this helps. :D

and when something is pinned - it stays in place rather than moving down the page if nobody has posted on it.