What do you mean, I will be his lock?

The meaning of this sentence varies from context to context, as follows:

1, this sentence may be a joke, and locking here can be understood as some kind of restriction or constraint. If it is a joke between friends, then this sentence means "because I am his friend, I will not restrain him."

This sentence may be an expression of promise or guarantee. In this case, the lock can be understood as protection or guard. If it is at a critical moment, this sentence means "As his friend, I will always protect him and close the door for him".