Summary

Contextual filters are like exposed filters that take the value they are filtering by from the URL. Even though they are filters, they are maintained in the third column of the view edit screen under Advanced. As an alternative to passing the value via the URL, contextual filters can create a default value from information that Drupal has about the environment that the view is being displayed in, such as the node ID if it's a page, the user ID if it's a logged-in user, and so on. This ability makes contextual filters very useful for blocks.

In the next chapter, relationships will be presented. Relationships are valuable when content has entity references, which include taxonomy terms. Without a relationship, all the content can display is the ID of the entity that is referenced by the entity reference field. However, add the relationship, and all these fields become available to use in sorting, filtering, and displaying as fields.