Online mode is a toggleable feature within any Minecraft server in the you can set it to true or false.

You can access the files either though:

File Manger ->
Utilities -> Server Properties

Setting it to true:
Setting online mode to true will allow only users with a premium Minecraft account to join your server.

Setting it to false:
Setting online mode to false both users with non premium and premium can join however this will have certain issues as you wont be able to see player's skins.

Changing this setting will cause player data to be reset.