If you click on your view page path, it tells you:
and as you can tell from the screenshot, it is still used in Drupal 8.
If you still do not understand, basically it is a place holder. Whatever the
% value is, it gets passed for your contextual filter.
Ex: view path of
When you go to
yoursite.com/hello/benia, it automatically passes the word
benia to your contextual filter.
/user/%/profile when you go to
/user/10/profile it atomically passes the
10 to your contextual filter, thus only the stuff related to User ID of
10 shows up.
You could go to
/user/sdlfkjlsadfklsdfja/profile and it passes
sdlfkjlsadfklsdfja to your contextual filter, but since this value is no good, nothing shows up in your view page.