With a double opt-in, someone actively opt-in, for example by registering via a registration form. Subsequently, the registrant will receive a registration confirmation with a link. By clicking on this link the registration is actually confirmed.
Does the registrant not click on this link? Then you may not approach this person by e-mail. That is why we recommend a single opt-in. You are of course happy with every newsletter subscriber, so if you forget to click on the confirmation link, it could cost you newsletter subscribers.
In the Netherlands, single opt-in is sufficient and double opt-in is therefore not mandatory, but this differs per country. In Germany, for example, it is mandatory to use the double opt-in method.