What does Ponyboy mean when he says " Johnny and I understood each other without saying anything"?

In the Outsiders by S.E. Hinton Ponyboy said this, because they have a close relationship. Anyone who has a best friend knows that at times they can finish their friend's sentences. They can almost get into their heads. When someone says something funny you laugh, because you know your friend is thinking the same thing. They understand each other on many levels. It's as if they are becoming a mirror image of you, which they may start to do. The saying goes "you become who you hang out with". Ponyboy and Johnny like each other so much that they start becoming like the other.