A whitelist is like a VIP list. It is sometimes called an allowlist. Someone on a whitelist may get early access to an NFT drop, reduced prices, or some other type of incentive. The project creators determine who is eligible, so there is no automatic addition to a list. Whitelists are being used as a way to reward loyal supporters of an NFT project and also promote a coming NFT release. They are often tied to the community that grows up around a project on a chat application like Discord or Telegram.
Want to know more? NFT Whitelist: 5 Ways to Get Whitelisted! (July 2022).