Haven't is a contraction, meaning it combines two words into one shorter form. It merges “have” and “not” into a single word that's quicker to say and write. You'll see an apostrophe (that little floating comma) where the “o” from “not” used to be.

We use haven't constantly in everyday speech and writing. When you say “I haven't finished my homework yet,” you're explaining that you still need to complete it. When your friend says “We haven't been to that park before,” she means that would be their first visit.

The word works with several pronouns: I haven't, you haven't, we haven't, and they haven't. Notice that it doesn't work with “he,” “she,” or “it.” For those, we use hasn't instead (which combines “has” and “not”).

Contractions like haven't make language flow more naturally. Compare “I have not seen that movie” to “I haven't seen that movie.” Both mean the same thing, but the second sounds more like how people actually talk. In formal writing, like essays or business letters, some people prefer writing out “have not” fully, but haven't works perfectly well in most situations.