It means the instance that is being created, in your case. In your head replace the "this" with "time", your SimpleTime instance name. Normally you wouldn't need to use 'this', except your constructor has 3 arguments that are called the same thing, so the part on the right of the = sign does not have 'this' because it's talking about the arguments, not the fields. This is called "shadowing", when arguments shadow fields. Normally, you wouldn't need 'this', except if it's shadowed by an argument, 'this' specefies the field. So
my question is, what is "this.hour" i mean, what is "this" used for?