The short review is "life's a bitch, then you die" LOL, that doesn't do this absolutely brilliant film any justice, though.
Imamura has crafted a rather sardonic, brutally honest mirror of humans. Like humans, it is both hard to watch, and simultaneously invokes emapthy. Few directors every achieve this level of mastery of the audience's emotions. It holds a well-deserved Palme d'Or.
The legend upon which the movie is based is called Ubasute (姥捨て) in Japanese, or "abandoning an old woman". There is no evidence of it ever being a common practice, though it's a persistent myth in Japan. One of the very plain statements of the director, is that Ubasute reflects the way contemporary Western culture treats it's elderly. In the USA, at least, nursing homes substitute for Narayama mountain.