It means that she is not in the mood to discuss whatever topic it is you are talking about. The smartest thing to do would be to just let her have some space and don't bug her about it.
If mean she/he wants you as a girlfriend/boyfriend
lots of love
It means your in buddy
That he likes you and want you to be his girlfriend.
she wants to be with you forever/ your relationship is forever
It means he will talk to you later. Guys say exactly what they mean.
It means she's not your girlfriend any more.
It means that he likes you a lot and would probably date you if he did not have a girlfriend.
he probably means that he likes you as a friend, but doesn't as a girlfriend
that probally means he can be your BFF or you girlfriend
She has seen a new side of you that she does not like.
She is two timing you.