Within a Player, the Concept Pane is the area on the right of the screen that (for Topics) contains the play mode buttons. If a Concept Page has been attached to an Outline Element (typically a Module, Section, or Topic), then the contents of this Concept page are displayed in this area, when the Outline Element is selected in the Outline. If no Concept Page attached to the Outline Element then, for Modules or Sections, the Concept Pane is empty.
For Topics, if no Concept Page is attached to the Topic, then the contents of the Start Frame are displayed in the Concept Pane, instead. (But see also the Show introduction text option in the Player Preferences for the Player and LMS deployment format during Publishing.)
What can you use as a Concept Page?
It is most common to use a Web Page as the Concept Page, However, it is also possible to use any file contained in a UPK Package as the Concept Page. this mans that you could use a Microsoft PowerPoint presentation, Adobe PDF document, or anything else as the Concept Page. However, this is not really recommended (by this author) as such files can take a while to open (and they will open as soon as the Outline Element is seleteced in the Outline), and are more often than not opened within the Concept Pane itself, so appear relatively small.
In UPK 11 and earlier, Web Pages were relatively lacking in rich formatting options (most notably, tables are not possible in UPK 11). If you need to use more advanced formatting you can create an HTML File in a Package, and then link to this as the Concept Page. This situation improved in UPK 12, which supports tables and nested lists, and also allows the definition of Skins.
However, it is important to note that the (relevant) document output types will include the Concept Page in the generated document only if the Concept Page is a Web Page. The content of any Package file will be omitted. Therefore, if you need to use a non-UPK document as the Concept Page, it is probably better to still use a Web Page, and include a hyperlink to the non-UPK document on this (along with the full, text URL so that print readers can go online and find the document if they need to).