When someone opts in to an email, they are expressing their explicit or implied agreement to receive emails from a particular sender or organization. The opt-in process is necessary to confirm that the recipient has given their consent to receive initial communications.
Implicit opt-ins are when a person makes a purchase from you, asks about a product or service you offer, or visits your website, all of which show that they are interested in receiving communications. In this case, the person's contact information is taken down, and it is assumed that they have agreed to be contacted.
In any case, it is important to make it clear to the person why you are collecting their contact information and how it will be used. You should also give them the option to opt-out of every communication.