This is perhaps the best and most famous part of the novel. Immeadiately following the rumble Dally and Pony run to the hospital to see Johnny. It will be the last time they see him alive. Johnny tells them that fighting isn't worth it. He understands now that it doesn't make sense to argue or fight. He has come to a new realization in his short life. He understands that no matter who "wins" or "loses' everybody loses.
Finally, Johnny repeats a line from "Nothing Gold Can Stay" and tells Ponyboy to "stay gold". He means that Ponyboy should stay true to himself. For example, Pony should continue to see good in the world, to notice colors, to watch sunsets, etc. He knows that these are the most important things in life. Johnny does not want Pony to get too "tough" or become cold and hard.
Right after the rumble ends, Dally tells Pony that they need to get to the hospital as soon as they can. Dally says that Johnny really wants to see Pony and that Johnny is likely not to live much longer. They rush to the hospital. When Pony tells Johnny the Greasers won the fight, Johnny tells Pony that fighting is pointless, it is no good.
His advice to Pony is to "stay gold." By this he means that Pony should not get to be a hard, angry person like some of the other Greasers are.