The following image displays GMPC’s main window:

I will explain the elements from left to right.

This shows cover of the currently playing album. If no cover is available, or the song has not information about it, an empty case or cd is shown.
Hovering the cover will show an enlarged version of the cover.
Right mouse click on the cover allows you to modify it.
You can modify the look of the cover art in the preferences pane. (With or without the casing)

Next to it is basic information about the song. This shows the Title, artist and album name.
If you click on the title, artist or album name it will jump to that section in the Metadata Browser.