‘The grass is always greener” is a proverb that teaches us it’s not good to be jealous (to want what other people have). It may seem like everyone around you has “greener grass,” meaning nicer cars, better jobs, etc.
But your neighbor probably thinks you have greener grass too, which means that your friends and other people think that you have better looks, a happier family, etc. So instead of thinking about what everyone else has, this proverb wants you to be thankful for what you have