Pressing the eye icon
will open or close a preview window showing the results of rendering the WikiText (the icon-eye will also open or close).
Right to the eye there is another icon, , which when clicked, shows you a dropdown which gives you access to six different types of preview:
output shows you the text as you see it when you do not edit the tiddler.
html shows you the source code of that output. Your web browser uses this source code to display webpages.
parsetree and widgettree show you the two object trees that TiddlyWiki internally generates from WikiText. The parsetree collects basic information about your WikiText. From the informations of the parsetree the widgettree then generates extended informations and finally the above html (and the widgettree also does other things).
differences from current shows you the differences since the tiddler was last saved. Deleted parts are red, added parts are green.
differences from shadow (if any) does the same, but this time it shows you the differences to the underlying shadow-tiddler.
By default, the preview pane button controls whether the preview pane is displayed for all tiddlers that are open for editing. There is a hidden setting to make the button work on a per-tiddler basis
The variable tv-tiddler-preview can be used to detect whether content is being displayed in the preview pane.