The phrase covers several different things
Opt-out clause is a loose term for any provision that lets a party stop. In practice the phrase covers at least four distinct mechanisms with very different reach, and a great deal of disappointment comes from assuming a contract contains one of them when it contains another.
The four are: termination for convenience, which permits exit on notice with no reason; termination for cause, which requires a breach and usually a failed cure; a break clause, which permits exit at a defined point; and non-renewal, which is not termination at all but a decision to let the term end naturally.
