The New Project option creates some topics for you automatically to get you started. In your normal daily work you will create topics yourself. Creating a topic in Help+Manual is very much like creating a new document in a word processor – but the document is also part of a larger project. Each topic in a project is actually an independent document, but you can manage all these topics as though they were a single document.
You will normally create new topics in the Table of Contents (TOC), which automatically creates both the topic file and an entry for the topic in the TOC, where you can move it around as you like. And you can also add text and other content to it, of course.
You can also create new topics in the Project Files/Topic Files section of the Project Explorer, but when you do this the topics will not have TOC entries – they will be "invisible". Topics created in Topic Files can only be displayed with hyperlinks, they are used for popup topics and for information you don't want to include in the TOC directly.
Add Topic/File (dialog reference)