Opt-in is a term related to data privacy. It means a permission given to marketers (or data to use the information about the user or perform other marketing activities.
Opt-in is a legal marketing approach when a marketer asks for users’ permission in order to send them promotional content. Opt-in can be full or partial, which means the user can agree to all or just some marketing activities.
Under GDPR, companies are required to ask for consent for sending any advertising messages unless they are processing personal data on the basis of legitimate interest.
Here’s a sample opt-in placed on the Piwik PRO website – an additional checkbox in a whitepaper download form: