When an email list is "opt-in", it means that subscribers to that list have stated that they want to receive promotional messages from designated sites or on selected categories. The list has essentially said, "Yes, I'd like to receive messages on the following..." They've given their permission on the front end, before ever having received one solicitation.
Email received based on a user's choice to opt-in, or rather their choice to be on a digital mailing list that (theoretically) is only used to send messages which will be of interest to them. Most opt-in email is advertising. Opt-in email continues to build market share in the online advertising world. More often than not, opt-in is the default and user action, such as a check box, is required to opt-out.