From WikiFAQs

A Topic is a collection of posts on a board. Topics are created on the main board page. Usually topics are about only one subject of conversation, and on boards with topicality, such conversation must only pertain to the intention of the board. Most social, dead, and special boards do not have such topicality restrictions.

On other internet forums, a topic is often called a "thread." On the GameFAQs message boards, topics are closed (automatically) when they reach 500 posts, but there are ways to get around this topic post limit.