It means that when the weather turns cold in autumn, grasshoppers will freeze to death. The extended meaning describes that the end of someone or something is coming, which is derogatory.
It is said that grasshoppers can't jump for a few days after autumn. If no one puts grasshoppers on the dining table, maybe the temperature will be warmer after autumn, and grasshoppers may be able to jump for a few more days after autumn. Now grasshoppers are eaten everywhere, and they are eaten only after autumn (note that as long as the season is right, all the creatures that eat are better in autumn), and the end of grasshoppers after autumn has really come.