Letters containing poems or expressions of love, and are not known by other people except the recievers mother.
Reciever is often a girl.
Secret love letters written by a girl are often not sent, and are not known by other people except the girl's mother or/and sister(s).
I sent a secret love letter to a girl (I'm a guy). Nobody knows about it.
I wrote a secret love letter to a boy (I'm a girl). I never sent it.