An opt-out customer is a customer who did not agree to be contacted over email, or unsubscribed from your emailing list.
Opt-out customers are difficult to convert into returning customers because you can't reach them through email and only hope they will return to your shop by themselves, or through another of your ads. Opt-in customers are very valuable to your business as they are reachable at very low cost through emailing campaigns, and therefore conceal a lot of potential for repeat business.
You can find out about your opt-in and opt-out ratio by looking at the Monthly Overview - Database Evolution dashboard. For your business to be profitable, Opt-in customers should represent at least 70% of your database, and Opt-out customers not more than 30%.