When opening the app you will be presented with the Arrange screen. The large grey area is where recorded and imported audio and MIDI clips are visualized and editing takes place. Audio is visualised as waveforms and can be arranged on different tracks. MIDI events are visualized as small rectangles, depicting the key and duration of each note. Each track has its own horizontal lane.
On the left side, the mixer channel of a track is displayed. If you do not have any tracks on the time line yet, this area will be dark grey. Only one mixer channel will be visible at a time. Selecting a track (horizontal lane) on the time line will make the corresponding mixer channel visible. When you want to display more mixer channels at a time, you can enter the Mixer screens by tapping the top-right icon.
The mixer channel has lots of buttons and sliders that can not be displayed on the screen at once. Therefore, it is divided into several mixer pages that can be accessed using the mixer page control at the bottom of the mixer channel. These are the round dots where the light grey one shows which mixer page is currently active.
The top of the app shows a row of icons for functionality that is used most frequent. When you switch to another screen, like the Mixer screens, other icons will be displayed that are applicable for the context.
The number of icons that are displayed depend on your screen size. Larger sized tablets will display all icons available, whereas smaller phones will likely display only the most important ones. The missing functionality of the icons that are not displayed is accessible through the 'More…' icon:
The functionality of the buttons is explained in the section The Arrange screen.
The app is also divided into different screens. You have the Arrange screen, the Channel mixer screen, the Bus mixer screen and optionally the USB audio hardware controls screen. These screens are accessible by using the top-right 'Next' icon. Every screen has this icon on the top-right, so you can flip through the screens by tapping on this icon one or more times. Eventually, you will arrive back at the Arrange screen.
You can read more on menus here.