There are two wave windows in the main screen. The upper window displays the waveform of the audio file always in its full length - it is useful for knowing where you are in the file, and you can tap it to quickly jump to a new area of the waveform. The lower window shows the waveform of the audio based on the zoom level. Tapping and dragging the waveform will create a selection.
You may also zoom in or out by doing a pinch gesture.