When you create a tag, WordPress automatically creates a slug from the tag name. WordPress makes some changes so that this slug is URL-friendly. For example, spaces are replaced by hyphens (“-“) and the slug is changed to lower-case.
For example the tag name
Redirect URL to Post
will automatically receive the slug
Additionally, WordPress makes sure that each slug is unique. In order to achieve that, it may add a number.
These slugs appear in some URLs, for example on your tag archive pages. Your visitors, however, won’t usually notice them. They see only the original tag name on the pages and in the page header.
You can edit a slug but WordPress will again make sure that it is URL-friendly and unique. When changing a slug, some links to that tag page might break and need to be updated as well. Usually there is no need to change a slug.
In some cases you will need to enter the slug in a shortcode. You find the slug in a separate column of the table on the “Tags” page.