Any time a user requests to be removed from any kind of online program, he or she is said to be "opting out." For example, if you no longer want to receive an e-mail newsletter, you have the ability to opt-out. Note that there is a difference between opting out and unsubscribing. You may only unsubscribe to something you have previously subscribed to, but you may opt-out of something you have never even joined in the first place. For example, if you fill out an online form to register or sign up for something, you may see a "yes" automatically checked in a radio button to indicate that you wish to receive something. Unless you manually uncheck the yes, you will be added to some kind of marketing list.
Opt-out also refers to a type of service that assumes inclusion unless informed otherwise. In modern slang, it's meaning has extended to refer to someone who is declining to do something, as in, "he's opting out of the party tonight because he has an early flight in the morning."