The friendzone is a subset of incelization. It means that you have been relegated to beta orbiter status and that the woman has put you on a Rolodex for possible future romantic situations. The term friendzone is often used a verb, as in, you've been friendzoned, and in practice may refer to situations wherein a girl says "I merely want us to be friends". Hetero women generally don't become true friends with someone they won't be willing to be romantic or sexual with later.
The Friendzone Myth
There is a myth that women don't fuck their friends and that dating is a measure of how much a woman wants to have sex with you. Truth is women fuck their friends if they are attractive. And generally women only form friendships with men they would be open to having sex with down the line. You don't go to your friend's sex parties? Yea cuz ur ugly. There's only a no-sex zone, which contains both partnered and non-partnered people.
Again, what most people mean when they say, "friendzone", is actually permanent sexgating, and at the end of the day has little to do with friendship.