When a person (usually a woman) knows that she has an admirer and decides to tell that person that she only wants a friend, then that person got friendzoned. If somebody was friendzoned, you can tell by the fact that the girl introduces that person with the words "he's just a friend" while pretending not to be aware of the elephant in the room.
There seems to be a book by Jane Austen, where a a girl called Elizabeth finds out that she has an admirer. Since she doesn't want to deal with that fact, she talks to her sister about what she could do with him instead. She makes the egoistic decision to make things convenient for herself and herself only: She decides to pretend that he's just a friend. The alternative proposed by her sister, which is to pretend that she likes him, is equally ridiculous.
Now some women think that Elizabeth seems to be a good role model. Often, the woman can feel wanted since the admirer continues to show her that he likes her, while the admirer suffers since he feels unwanted.
Since she friendzoned him, she just keeps on pretending that he's just a friend, even though he told her several times that he likes her.