In terms of email security, the act of whitelisting refers to creating a list of contacts that are exclusively allowed by the account owner to send messages into his mailbox without any impediments whatsoever.
Meanwhile, everyone not included in the whitelist is automatically blocked.
This is the complete opposite of how a blacklist works; i.e., blacklisting involves creating a list of banned email addresses that are suspected of spamming, such that all the messages from these people are filtered away even from the server level.
Standard-issue spam filters included in popular free webmail like Gmail and Yahoo Mail usually sport both whitelists and blacklists of the senders as well as keywords to look for in messages in order to assure prudent spam elimination without accidentally condemning legitimate emails into the deletion queue.
An email filter's whitelist allows all the listed contacts there—which should include details like IP addresses, email domains, and email addresses—to send mail to your inbox without any restriction whatsoever.
There are also Internet service providers (ISPs) that use whitelists in order to manage and sift through email to be delivered to their consumers.
It's a service that enables the ISP to always deliver the mail of legitimate senders without the chance of inadvertent blockage care of overzealous spam filtering.
In fact, ISPs typically receive quite a lot of requests from valid businesses in order to get added into their whitelist.
If the ISP's whitelist is exclusive, then no other mail would be allowed to get through the customer's inbox save for the addresses indicated on the special register.
If the catalog isn't exclusive, then the filter is forbidden from automatically sending messages to the spam folder or getting them deleted.
By and large, only the account owners would ever set their spam filter to the exclusive whitelist filtering mode, because most ISPs or email services respect the customer enough to give him control over the option to delete all messages from sources not found on the whitelist.