The first time you open Cullibrate, the guided setup asks how you work and then introduces the workspace. You can replay that tour later from Help.
The workspace
The centre pane is where you review photos. Grid, Filmstrip, Compare, Survey, detail zoom, and Face Survey each answer a different culling question. They are covered in Culling basics.
The ingest bar
The top bar is how photos get into Cullibrate. You can Open in Place (work from an existing folder) or Copy From Card (bring photos onto your machine first).
Your keyboard starting point
Choose Cullibrate defaults or a shortcut preset based on another photo app. This changes the keys you start with; you can remap them later in Keyboard preferences.