When using a single opt-in, customers will only have to text message your keyword to your short code to subscribe. When using a double opt-in, new subscribers will text message your keyword to your short code, but before they're subscribed, they'll have to respond to your request for information (first name, zip code or email address), or request to confirm their opt-in (Y or Yes).
To learn how to setup a single opt-in, click here.
To learn how to setup a double opt-in, click here.
If you're adding new subscribers through your website, it's required that this be a double opt-in process.