Opt-in refers to the practice of obtaining the explicit consent or permission of a user to receive communications or services from a business or organization. In the context of online advertising, opt-in typically involves asking the user to provide their email address or other contact information, and to agree to receive marketing or promotional messages from the advertiser. Opt-in can be an important part of an advertiser's marketing strategy, as it can help to build a database of qualified and engaged leads, and to deliver relevant and valuable messages to the user. Opt-in can also be beneficial for users, as it can provide more control and transparency over the communications and services that they receive.