Also known as an allow list, with the Internet, a whitelist most commonly refers to a list of websites allowed to display ads to its visitors. With more and more users installing ad-blocking programs, many sites deny access to visitors until they turn off these programs. In other words, the users must whitelist a specific site.
Additionally, a whitelist may refer to a list of senders that an e-mail blocking program allows through a spam filter or another e-mail filter.
Because "blacklist" and "whitelist" is offensive, it's best to refer to this type of list as an "allow list."