"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 him had he acted differently when they first met. -see example
-being "friendzoned" does not have to mean something negative in nature, it is simply a word to label the common situation of a male is only seen as a friend in the eyes of a female come the time he confesses his feels for her, which is completely ok, by no means does the male friend feel he is intitled to be with said female(unless they are an assholes), everyone obviously has the right to reject who they wish, but that does not mean the male has no right to feel rejected and inadequate, especially in situations where said female chooses to be with another man of which the male friend feels is not appropriate for said female, for whatever reason, such as opposing males personality(taking advantage of her and such). its mostly based on social norms and essentially stereotypes of male and female interaction. Of course this is not always the case, it is possible for a female to become "friendzoned" as the roles are simply reversed, though it is less common as guys are more in the position of asking a girl to be with him (a social norm). I personally dislike like the reality of social norms, but that does not mean they are not blatantly obvious, more so when observing rather than participating.