Topics are a visual representation of tables in a database. They are logical groupings of related information about a particular facet of your business, such as Customer or Sales. A list of topics is shown in the Catalog list
A topic item is an individual items in a topic or metatopic.
You build queries by adding topics from the Catalog list to the Request pane. You can also drag and drop any topic item to the Request pane. When you refresh a query, data for all the topic items present on the Request pane are returned.