Your community forums in Freshdesk are organized with a 3 level hierarchy:

Categories --> Forums --> Topics

At the top level, you could have various community Categories, within which you could have specific Forums. Each of these forums can have as many Topics of discussion as you need. 
Topics or individual posts in your Forums are the actual discussion threads. Based on the type of forum, users can vote up feature requests, questions or problems inside a specific topic. You also have the option to link a specific forum topic to a ticket in Freshdesk.
Forums are collections of similar topics, so customers know exactly where to go to get a specific answer from the community. For example, you can have a specific forum to collect Feature Requests from your customers, another forum for the community to share tips and tricks and a third forum where customers can talk about the problems they are facing and their work arounds.
Finally, you can organize your forums into specific Categories. In addition to helping you organize your community discussions better, Categories are useful when you are providing support across multiple products. You can choose to only show certain forum categories pertaining to a specific product in that product's portal. All other forums will not be shown to the customers, when viewing the forums tab in this product portal.