"Swing", in balance terms, is term that refers to the effects of chaotic factors that cannot be balanced for. A classic example of a swingy role is a Vigilante. A Vig that only kills Townies is a giant detriment to Town; a Vig that only kills scum can win the game for Town even without any "correct" eliminations. However, until the Vig role is assigned to someone and they start forming reads, it is impossible to predict where between those two extremes the Vig will perform.
Note that swing is an inherent part of the game simply based on which people are playing the game and who draws which alignment - you can't balance for three newbies drawing scum and ten experienced players drawing Town, because that's entirely outside the setup itself.
Swing is not generally considered a good thing. If through a combination of unfortunate events a Town or scum team winds up in a nearly unwinnable position, players in the offended faction may feel bitter that the setup swung against them to the point where they lost through no fault of their own. Thus, when designing games it's important to make sure that if the game can swing against one faction, it can also swing against others as well. It is also important to make an effort to design games so that players' welfare generally come directly as a result of their own actions, rather than the actions of others. This is especially true of large games with many power roles, which are notoriously difficult to balance.