This is the short form of not, but it is not a free word you write on its own. You clip it onto a helping verb, and the apostrophe shows that a letter has been dropped — usually the o in not: is not → isn't, do not → don't, have not → haven't, could not → couldn't. This is how English actually sounds in speech, in messages, and in almost any writing that wants to feel like a real person talking. Very formal notices still use the full not sometimes (do not enter), but n't is not slang; it is the ordinary living shape of not. Though actually, a few forms refuse to be tidy: will not becomes won't, not willn't, and cannot becomes can't, with an extra letter already gone. Ain't is a different, much more informal (and often judged) form — it is not the regular n't pattern teachers mean here.