Isn't is a contraction, a shortened combination of two words: “is” and “not.” When you squeeze them together and replace some letters with an apostrophe, you get isn't.

You use isn't to say that something is not the case or is not true. “That isn't fair” means the same thing as “That is not fair,” but it sounds more natural in everyday conversation. “She isn't here yet” tells you someone hasn't arrived. “This isn't working” means something has stopped functioning properly.

Contractions like isn't make English flow more smoothly when speaking. Compare “It is not raining” with “It isn't raining.” Both sentences mean exactly the same thing, but the second sounds like how people actually talk. When you're writing something formal, like an essay for school, your teacher might prefer “is not” over “isn't.” But in regular conversation, stories, and casual writing, isn't feels perfectly natural.

Other contractions work the same way: wasn't (was not), aren't (are not), haven't (have not), and didn't (did not). The apostrophe shows where letters got removed when the words merged together. Think of contractions as a kind of verbal shortcut that English speakers take dozens of times every day without even noticing.