Opt-in is a term used in email marketing to describe a process in which a person gives their explicit permission to receive emails from a particular sender or brand. In other words, an opt-in is when someone chooses to sign up for your email list and agrees to receive emails from you. This is in contrast to an opt-out process where someone is automatically added to a list and then must choose to unsubscribe.
Opt-in is important in email marketing because it helps ensure that people are receiving emails they want and are more likely to engage with. It also helps maintain compliance with anti-spam laws, such as the CAN-SPAM Act in the United States and the GDPR in the European Union, which require senders to obtain explicit consent before sending marketing emails. Opt-in can be achieved through a variety of methods, such as sign-up forms on a website, in-store or event sign-ups, or through other means of explicit consent.