When you visit a website and see "Enter your email for our newsletter," choosing to subscribe is opting in. Clicking "Unsubscribe" at the bottom of an email is opting out. Both are fundamental to ethical and legal email marketing.
There are two types of opt-in: Single Opt-In (subscribe and immediately start receiving emails) and Double Opt-In (subscribe, then confirm via a verification email). Double opt-in builds a cleaner, more engaged list. Opt-out must always be available — it's legally required in most countries. An easy unsubscribe process actually helps your deliverability because it prevents frustrated recipients from marking you as spam.