Stay informed and remember posts worth revisiting.
Watch a thread: click Watch at the top of any thread — you'll be notified when anyone replies.
Watch a forum: click Watch on a forum page — you'll be notified when a new thread is posted there.
Unwatch: same button toggles off.
Bookmark a post: click the bookmark icon next to a specific post. Bookmarks are private to you and appear at /bookmarks.
Difference: subscriptions/watches push notifications to you. Bookmarks are silent — just a personal "come back to this later" list.