The -2 means that a couple of people (out of an unknown number) didn't like something. For example, some people don't agree with you and think that it's never appropriate to use foul language.
Most of the time you get downvotes on something, the number of downvotes is small. Only the cases where something is massively downvoted and flagged out of existence can be readily explained.
Suppose 5000 people look at your posting and almost all of them agree that sometimes it is appropriate to swear, and your posting is one of those occasions: but two out of those 5000 don't agree, and downvote you. Well, there you go: you have the tacit support of 4998/5000 readers (just not enough to give you an upvote for it) and two downvotes.
Being acceptable to the majority in some "common sense" way is not enough to spare your comment. Either your comment has to have appeal so that it attracts upvotes from a significant segment of the site readers who come across it, or, failing that, a comment which doesn't attract upvotes, has to avoid raising the ire of minorities.
(Regarding the use "minorities": I'm using that here to denote absolutely any rare trait, background or world outlook whatsoever (where "rare" doesn't mean "odd" or "strange"); I am not specifically referring to "ethnic minorities".)
For the above reasons, situations where a comment is reduced to 0, -1 or even -2 are not always easily explained, other than by the general remark that it is probably less than "spotlessly inoffensive". (Imagine the ways in which some element in the comment might rub someone somewhere the wrong way, and you have a halfway plausible explanation.)
Only when some comment is massively flagged out of existence is there an obvious plausible explanation.