The default route's URL first points to the name of the Controller, the second segment of the URL points to the action of the Controller and the third segment points to the id which may and may not be passed in the form of paramenter of the action.
The default value of the controller will be Home, for action it will route to Index action and by default it takes Id as empty string.
For example:
/Home/Index (with id as empty string)
/Home/Index/7 (with id passed as 7)