Double Opt-In is an email subscription process where the user confirms their signup through a follow-up confirmation email.
It matters because email is one of the few channels a business can own directly. Strong email fundamentals improve trust, deliverability, segmentation, and long-term lead nurturing.
Example
A subscriber enters an address and then clicks the confirmation link in a follow-up email. The list treats the address as confirmed only after that second action, reducing accidental or mistyped signups.