This applies on the Web preview on the Articles main tab.
Since an article can be displayed on several pages (front page, index page, full text page, etc), a list of view options in the toolbar of the Web preview gives you a possibility to taste all such appearances, and to adjust headlines and similar at the best.
The list of options depends on your customized configuration, but generally it offers variants of article layout on a Web page, e.g. left or right , as lead, or as filler, with or without image and similar.
See also