If you choose an item from the Generate menu when there are no tracks
in a project, a new track is created. Otherwise, the current track is used.
If a track is selected and the cursor is placed in a single place in the track audio is inserted at the cursor position. The default duration is 30 seconds. The audio created will replace the any selection, otherwise it is inserted into the track, shifting up later parts of the track.
If a region of audio is selected, audio is inserted at the left
edge of the selection, and the length of the inserted audio is equal
to the length of the selection.
Silence - inserts silence
Tone... - you can create a Sine wave, Square wave, or Sawtooth wave.
White Noise - inserts random audio samples, which sounds like
Any items which appear after these three built-ins are VST, Ladspa, or Nyquist plug-ins. It is possible for a poorly written plug-in to crash Audacity, so always save your work before using a plug-in. Note that any effect that doesn't take any audio as input will automatically be placed in the Generate menu.