What is percent sign in the arguments? The percent sign is something I once saw in Drupal 6 and Drupal 7, but not  Drupal 8. It has been a few years ago; I don't recall where exactly inside a view, but most probably inside the contextual filter criteria.
What is percent sign in the arguments?
The percent sign in view arguments is to tell Drupal to expect a contextual filter within the view path. It is specific to Drupal 6 and 7, linux doesn't matter.

Drupal 8 uses curly braces to accomplish this instead.

From https://www.drupal.org/node/1578576, the example provided is that user/%/comments would provide a contextual filter to show the comments for the specific user.

This resource gives screenshots and examples about how to create a contextual view filter:
https://www.drupal.org/node/1578564