"Friend zone" describes the way most women stop seeing a male friend as dating material after a certain amount of time has passed, even though she might have begun dating them had he acted differently when they first met.
John meets Mary. Mary thinks "This guy seems nice. He's quite funny and reasonably good looking". John doesn't ask Mary out, but they see each other at social functions. After a while, Mary stops seeing John as a nice guy who is quite funny and reasonably good looking, and just sees him as friend. John realizes he's secretly madly in love with Mary and begins trying to hang out with her more and more. Because John loves Mary, he stops behaving reasonably and starts doing anything just to spend time with her. Eventually, he summons his courage and confesses his love for her. But Mary just feels uncomfortable and confused. She doesn't want to lose her friend and she feels pressured by his feelings for her. To be honest, John's been a little needy and pathetic lately, hanging on her every word and catering to her every whim, and that's not really very sexy.
John had a chance when they first met but he fucked it up by not doing anything about it. When he finally got his act together, the friendship that developed actively interfered with his potential of getting Mary to go on a date with him.
source & further examples: reddit.com/r/TwoXChromosomes/comments/o01v8/what_i_think_when_someone_talks_about_friendzoning/c3denue
person A: I tried to kiss her, but then she said " I think we should just stay good friends. "
person B: Wow. You totally got into the friendzone.