1. to choose not to be part of an activity or to stop being involved it

Within any society, there will usually be people who decide to opt-out (= choose not to live the way most people do)

Employees can choose to opt-out of the pension scheme.

2. a situation in which some members of a group choose not to join or be involved in an activity

Since the opt-out, the hospital has been responsible for its own budgeting.
The problem for Facebook is that Apple's update is about to educate a lot of users on how much Facebook is actually tracking them--and it's going to make it a lot easier for those users to opt-out.