The signature is only visible to you. What does this mean?

What a boy says can only be seen by you, which means that he just wants you to know his thoughts, care about you more and treat you differently from others.

If a boy just talks about it every time and only you can see it, it proves that he really cares about you and wants to show you his thoughts, but he is too embarrassed to send them directly to you, so let you know by talking about it.