The whitelist decides who can join and who can't. Turning the whitelist on makes it so only the people in the whitelist file can join.
Toggling the whitelist: /whitelist on turns it on, and /whitelist off turns it off.
Adding people to the whitelist: /whitelist add PlayerUserName So for example, if your username was OwnageHosting, you would do /whitelist add OwnageHosting
Removing people from the whitelist: /whitelist remove PlayerUserName So for example, if your username was OwnageHosting, you would do /whitelist remove OwnageHosting
And that's it! You don't even need to edit the config file or restart!