Whitelisting is the opposite of Blacklisting. A Blacklist is a list of players that cannot join your server. (This pretty much means they're banned) A Whitelist is a list of players that can join your server, but only those specific players can join. It's up to the Host of the server (I believe admins and mods can do this as well) to manage who can be on the whitelist and how to take requests.
Hope I could help!
- Sharp