Page displays have a path and an optional menu component. Page displays will be the primary content for the page, meaning they will be displayed in the main content area when you visit the URL that corresponds to the path.
Page displays take their arguments from the URL. You can embed arguments into the URL using %; in previous versions of Views, this was '$arg'. For example, 'node/%/foo' will accept URLs such as 'node/1/foo'.
Please remember that using a % placeholder makes the argument required. If you wish to have an optional argument, simply omit the % from the path. I.e. using "page/%" as the path requires an argument and visiting 'http://www.example.com/page' will not trigger the view.