It is a figure of speech, usually not meant to be taken literally. It means that the speaker would possibly be willing to perform an act that goes against their moral code to obtain the object of their desire. In most societies, murder is considered the worst possible immoral action, above rape, armed robbery, or treason, and so the phrase usually starts off with "I'd kill for..."
In other words, they would perform the most immoral deed they could think of, murder, if it meant they could achieve their desires. Often times, such desires are either based on survival instinct ("I'd kill for a glass of water") or ego ("I'd kill for those shoes"). In most cases, murder is not necessary, as the object of desire is usually within the means of the individual, and in the remaining cases, murder usually wouldn't achieve the desired goal anyways-- one cannot physically change their bone structure, become a billionaire, or become famous (perhaps infamous) by committing murder.
Especially when spoken in cinema or television, the statement is almost always figurative, since most characters have some sense of morality that would prevent them from actually committing murder intentionally, and most people in civilized society would feel the same. Most people have a strong enough sense of morals that they could not literally carry out the act of murder, even if someone offered them a natural or supernatural way to attain their desire.