-- The wavelength and the amplitude were described in my answer to your previous question.
-- A "compression" is a place where the wave is <em>compressed</em>. It's the darker section of the wave in the picture, where the wavelength is temporarily shorter, so several waves are all bunched up (compressed) in a small time.
-- A "rarefaction" is exactly the opposite of a "compression". It's a place where the wave gets more "<em>rare</em>" ... the wavelength temporarily gets longer, so that several waves get stretched out, and there are fewer of them in some period of time. The arrow in the picture points to a rarefaction.
