It means you’ll always be friends, no matter what.
The lady I’m dating, we’ve each expressed many times to each other that no matter what happens with our relationship, we’ll always be friends. And we mean it. She could run off with my car and my best friend, and my only response would be to ask to be invited to the wedding. I could be so mean to her that it makes her cry for a week, the day after we’d go back like it never happened.
It means your bond is deeper than the momentary things we want and need. It means that you’re so comfortable with each other that you can talk about anything. It means that no amount of space will keep you from making time to see each other. It means knowing someone so well, that the very idea of not trusting them for something just seems so outlandish as to be unthinkable.