I always like to think of it in terms of programming where a new thread allows a process to run its course separate (sort of) from all of the others.
You can launch a process on a new thread, much like you create a new thread on a message board.
Each thread ideally serves one purpose, much like a message board thread serves one discussion topic.
Threads can be joined when necessary, much like message board threads can be merged.
Threads can be locked, much like message board threads.