The 'friend zone' is the situation where one person wants a romantic relationship with another, but the other only wants friendship. Popularised by the 1994 Friends episode 'The One with the Blackout' (Ross is in the friend zone). The concept has been criticised by feminists for framing friendship with a woman as a punishment for not getting sex. If your teen uses it, it is mainstream relationship vocabulary; worth a calm conversation about what healthy friendships look like.