A help topic is a separate document in a user guide or software manual, which is created using a modular approach often called “topic-based authoring” in the technical communication area. If you think about an ordinary document created with office text editor, the notion of a topic maps to the notion of a “section” or a “sub-section”.
Typically, a help topic is a pretty small chunk of content that focuses on a single subject and can be consumed separately from the remaining part of the same user manual. Another difference is that an office document is linear, while a topic-based manual is a non-linear set of separate documents (help topics) with links and navigation. A nice benefit the topic-based approach gives you is the ability to re-use topics in multiple places of a user manual, as well as assembling several different outputs from the same topic set.