Chapter 9 : What did Johnny mean when he told Ponyboy to "stay gold"

Johnny encourages Ponyboy not to waste his youth looking for things he does't have. He tells Pony that life is fleeting and nothing is permanent hence "Nothing Gold can Stay". Johnny encourages Pony to see the goodness in people like Darry instead of wasting their relationship. He wants pony to stay innocent and good as long as possible.