That word is all the way negative and offensive. There is nothing "positive" in its meaning. Even its mentioning on this forum may bring about some strong reactions if not offend some people. Some people may handle it better than others, but that does not mean one could use it gratuitously.
Okay, now that we got that out of the way, I will get off the soap box and take a short look at what this terrible word is about.
Simply put, it just means "to have sexual intercourse". Well, there is nothing wrong with that on its own, but "fuck" has a strong connotation of having sex with disregard to your partner, in a objectifying or even demeaning manner. In other words, it is all about the subject of the action and his/her (yeah, it is gender neutral, although slightly biased towards men) satisfaction or need to self-affirmation often at the expense of the person on the receiving end. (To be fair, "fuck" does not exclude the possible enjoyment of the act for the receiving person, but that enjoyment is usually dependent on how that latter person handles the position she/he is placed in... well most of the time.)
Well, the meaning above is the basic one, and arguably the closest one can get to a neutral meaning. Then, we only go down the drain. Here, one will dsicover a plethora of meanings and usages, so I will just mention, "fuck off", which means "go away" in a very offensive way, or "fuck up", which is pretty much somewhere between "mess up" and "destroy", or "fucking", which is an expletive and does not really bear much meaning besides a vessel for one's frustration or anger, or even simply "fuck", which again is an expletive efficiently funneling one's outrage against fate or whomever in a simple four letter word of utter power. And one can go on and on....