You can go through life and make new friends every year - every month practically - but there was never any substitute for those friendships of childhood that survive into adult years. Those are the ones in which we are bound to one another with hoops of steel.

This quote is saying that making new friends as an adult is great, but it is not the same as keeping good friendships from childhood. Friendships from childhood that last into adulthood are special because they have endured many years of change and difficulty and are very strong, like steel.