What exactly is a "thread" when posting to a forum on a site? Not sure exactly how to phrase the question. Maybe just looking for the definition of a "thread" as it relates to the Internet.

Yes, it's forum specific jargon.

Imagine that the top level of forums is a long stick, this contains all the categories.

Then you have a number of threads tied to and dangling from the stick, these are each question and it's replies.
Each of these are a thread.

If all the messages were posted in chronological order, rather than grouped in threads, nobody would be able to follow a discussion. Threading allows the discussion to be split into groups where all messages are related.

Also see:
Threaded discussion - Wikipedia, the free encyclopedia