Contextual filters in Drupal allow you to add or remove information (content) to the currently displayed view. The filter acts dynamically based on the URL.
If the provided argument value is in the URL, the view will contain certain results, if the value is not in the URL, the view will be filtered with other results.
This whole concept implies certain complexity, so the best way to clarify it is with an example.
Keep reading if you want to grasp the idea behind contextual filters in Drupal 8!