edit: whoops I forgot about the incel conception of it, that if you're friendly to a woman she'll put you in the "unfuckable" box. Anyone who has dated a friend knows that just isn't true lol.

This is a struggle session tier controversial topic, but "friend zone" is what happens when

(a) you don't want to date someone, but are afraid to reject them completely. A lot of people soften the blow by saying "oh we can still be friends", in the lying polite way you'd say "we should hang out more!" before never following up on it. Especially prevalent among women, because men are dangerous, and especially prevalent when you don't want to rock the boat in a friend group. It's actually quite difficult to be friends with an ex or failed romantic prospect even if both people genuinely do want to be friends; you have to manage strong emotions without being able to directly change them.

(b) the rejected party either doesn't understand or refuses to accept the rejection. Classically, this leads to men trying to "win over" women who don't want them, and honestly probably don't even want to be friends now that the dude is being weird about it, while thinly pretending to just be a good platonic friend. Let me get that door for you mlady.

It's totally legit to want to date someone but not be "just friends" with them. It sucks to lose a friend that way, it's happened to me, but we're all adults here and sometimes people have enough friends already or don't want to be friends with you badly enough to deal with any additional heartache from working through those emotions.