There are some threads in each forum which are special. These are Stickied threads, Important threads, and Locked threads.

Stickied threads are stuck at the top of each subforum. They'll never move down the list like the other topics. They have a thumbtack icon next to them. Usually a thread is stickied because it has important information (like rules) or is a huge discussion topic or ongoing project.

Important threads are highlighted with red text to make them stand out. They'll move down the list like other topics though.

Locked threads mean that one of the moderators or admins has decided that there's no useful discussion to be had in a thread (for various reasons) and so you can read it, but you can't reply to it. Locked threads have a padlock icon.

Here's an example of a thread which has been stickied, important-ed, and locked, above a normal forum thread: