Double opt-in adds a confirmation step: someone signs up, gets an email, and must click to confirm before joining your list. Single opt-in adds them immediately.
The trade is size versus quality: double opt-in loses the fraction who never confirm, but everyone remaining is verified-real and demonstrably willing — better deliverability, cleaner engagement, and stronger consent records (required in some jurisdictions, standard practice elsewhere).
Our default: double opt-in for anything public-facing (forms bots can find), single acceptable for closed contexts like customers you already serve. Bot-stuffed lists poison sender reputation fast enough to make the choice for you.