What did the boy tell you about his door lock?

It means that he likes you and trusts you. Maybe I hope to have your company in the future, so I tell you the lock password like this. Of course, it is also possible that his personality is such a casual person. If he thinks this person can be trusted, he will tell others the password. I believe he likes you and is familiar with the feeling of trust that can treat you as a particularly good friend.