-- The wavelength and the amplitude were described in my answer to your previous question.
-- A "compression" is a place where the wave is compressed. 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 "rare" ... 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.