Tagging of content is a great and easy way of providing structure and extra semantic meaning to your content. It allows sites with lots of content to provide relevant search, filter and navigation options. For instance, you can lift articles as "relevant articles" in article pages based on the tags.
How to view and create tags?
You can view existing tags and create new ones under "Tags" tab in assets folders.
First select asset folders:
Then open "Tags" tab to view the tags:
You can create new tags, for example under the Themes folder by clicking from the 3 dots next to the folder and then "New Item".
How to add tags to a page?
You can add tags to almost all page types. Usually there is a tag area in the content tab of the page:
Or if you are creating an article page you can filter related articles by adding a tag under "Related articles" section. You can read more here.