Isn't is the ordinary short form of is not. You push is and not together, the o in not drops out, and an apostrophe marks the missing letter: isn't. It is the form people reach for in speech, in messages, and in writing that is meant to sound like speech. You also hear it at the front of a checking question: Isn't this yours? which is less a request for new facts than a way of saying I think this is yours — tell me if I am wrong. Though actually, school English still treats the full form as the polite one on formal paper, so the same student may write is not in an exam and say isn't to a friend two minutes later. Older grammar books call the short form speaking-only; younger writers type it everywhere, even in quite careful emails. It still means exactly is not. It is not a softer word, and it is not a different verb. One more real-life wrinkle: with I we say I'm not, not I isn't, and in some places you will hear ain't instead — teachers mark that last one as non-standard, but you will meet it.