Opt-in is the expression of data subjects’ permission or consent authorizing the third party to contact them further and/or process their personal information that was given to the data controller.
According to the GDPR, opt-in consent is „any freely given, specific, informed and unambiguous indication of his or her wishes by which the data subject, either by a statement or by clear affirmative action, signifies agreement to personal data relating to them being processed“.