Hi Rob
Glad you are finding Stroll useful.
The section above the horizontal line is the text field of a tiddler. That is the main writing area of the tiddler, and where you would place links to any other tiddler. When using the newhere (+) button, the new tiddler created places a link
to the previous tiddler in the text field of the new tiddler.
The section below the horizontal line is a references section listing any backlinks to the present tiddler, so you can see topics that are related to the present topic. You can view those other tiddlers as links (default), as search results showing the context of the backlinks in those other tiddlers, or as full transclusion of the text field of those tiddlers.