There are a few options. If you want the text to appear right below the topic name, you can add text to the topic summary. Underneath the topic name click the Edit Section button. Enter text, images, links, etc. in the Summary box. Click Save changes.
Another option is to add a Label. Please see our help guide on adding labels. Labels are best for short bits of text.
You can use a Page (which is collapsible/expandable) for text, links, images, embedded videos, etc. Here is our help guide on adding a Page.