A single opt-in is when someone has actively subscribed to your newsletter, but does not need to confirm this registration via a link in a registration confirmation, such as with a double opt-in. A registration confirmation is sufficient.
The advantage of a single opt-in is, if someone forgets to click on the link to confirm the registration, you can still contact this person by e-mail. This is also the reason that we recommend a single opt-in instead of a double opt-in. You are of course happy with every newsletter subscriber so if you forget to click on the confirmation link, this can cost your newsletter subscribers.
Legally speaking, a single opt-in in the Netherlands is sufficient. The rules may differ per country, because in Germany, for example, double opt-in is mandatory.
With a single opt-in, it is important to set up a registration confirmation with an unsubscribe link. After registering for the newsletter, someone will receive a registration confirmation with the option to unsubscribe. If someone is registered by another person, he can cancel his registration in this way.