What does ponyboy mean when he says johnny and i understood each other without saying anything?

When Ponyboy says this phrase he means that they don't need to say words to share the problems they have in their life. Both of them have problems so they understand each other like no on do : Johny's parents do not pay attention to him; Pony must live his life with a big loss - death of his parents. So they both know how life can be unfair sometimes.