Controlling who can join — and who can wake your server
The whitelist does more here than it does on a normal server.
Turn the whitelist on under Access on the server page, then add Minecraft usernames.
On a free server the whitelist does something extra: it is checked before the server wakes up. Anyone who knows your address could otherwise start your server whenever they liked, which uses your slot and everyone else's capacity.
A whitelist that is switched on with nobody on it will stop you from starting the server too. The server page warns you when that happens.