Opt-in simply means that someone has given their permission to receive emails from you. They can do this by checking a box when they sign up for a website, or by filling out a sign-up form. The process can be a single opt-in or double opt-in, however double opt-in is the best practice to follow.
If you only give a subscriber one opportunity to give their permission, you can get misspelled email addresses or they could change their mind. Sending them a welcome email that asks their permission again is the best way to avoid that. By following this practice marketers can keep their email lists clean. Opting in may seem like an extra step, but it’s worth it to make sure you have accurate contact information and that your subscribers are genuinely interested in hearing from you.