Sample edit mode allows you to modify individual audio samples using your computer's mouse. It is useful for manually editing out clicks and pops from noisy recordings.
To edit a sample, zoom in on the audio waveform until the individual samples become visible (they will appear as vertical lines). Click on Sample Editing Mode from the menu or select the pen icon from just below the waveform and edit the amplitude of a sample by holding down the left mouse button and adjusting it's height.