Usually two levels of navigation entries are placed at the top of the shell. The folders are on the first level, and directly below are the entries for the second level of navigation for the selected folder. If a folder is flagged as single top level, then only the root folder is displayed at the top. The second level navigation entries are moved to the left side of the shell. This option is available, because for some types of roles, it is better to place the complete navigation on the left panel and not use a second level top navigation.

(Single Top Level)

You can set this flag on the Menu tab in PFCG if you select the relevant folder and choose Other Node Details.

Be aware that the checkbox for Single Top Level is only shown at the root folder level.