If you know what you're looking for, you can use the search bar. If you want to explore a bit more, you can get to the topic map by clicking the map pin icon.
The topic map shows all of Expii's topics and how they fit together. Specific topics nest inside more general topics. Think of it like a textbook's table of contents: the whole book has a topic, and each chapter is a little more specific, and chapters might be divided into even more specific sections.