There's always a debate in linguistic circles about the exact meaning of fluency and whether or not it's even useful in evaluating a language learner's progress. To some, fluency implies a high-level of knowledge and skill with a language, someone who knows a lot of the grammar and vocabulary and can apply that in a conversation or writing or whatever. To others, it has more to do with the actual delivery of the language; do you pause a lot? Is there a lot of hesitation or false starts and self correction? If so, that person might not be fluent in the eyes of some. Other still are less concerned with being "grammatically correct" in a language and instead focus more on whether or not the meaning was expressed and whether or not the listener understood what was being said; that is to say, whether or not the speaker can effectively communicate regardless of grammatical accuracy. I would think of it as a combination of these ideas and I personally am more concerned with communicating effectively (read: not necessarily "natively") and I like to have a nice flow to my speech without too much hesitation or correction, but I think that second point is less important than the effective communication. I hope this made sense I kind of rambled.

Fluent means you can speak smoothly without stopping to think of words to say. If you keep saying, uh, ah, um, then you are not fluent.

The verb "fluere" (flow, stream, pour, be soaked) in Latin refers the continuous motion of all kinds of liquids and gas. Cognate with the English word "flow". The present active participle of "fluere" is FLUENT* (flowing, streaming). This word was originally applied not only to liquids but also to the way of speaking and thinking: when someone speaks fast and clearly, it's said that his way of speaking "flows" like a river. The rapper Guru says in a song: "free with the style / I flow like the Nile / but remember / don't mistake the smile..." So a fluent person is someone that speaks very well: For example: "She's fluent in Russian".