When the white list is enabled on a Minecraft server, only players you give access too will be able to join. Setting up a white list is easy, just follow this tutorial and you will be playing in no time.
- Make sure your are OP on the server
- Run: /whitelist add <username> for each player you want to white list.
- When you have added all the players run: /whitelist on
- The white list is now active and only those on the list can connect.
- If you need to remove a player from the list run: /whitelist remove <username>
You have now enable the white list and your server is protected from unauthorised players.
As always, if you still need help make a post on r/MinecraftHelp where one of us will be happy to talk you through it.