What is a Tag?
A WordPress tag is a default taxonomy that enables you to categorize your posts. Readers can then click on a post’s tag to view similar articles with the same tag.
More About Tags
You can add as many tags to a post as needed. However, tags are an optional feature. Therefore you do not need to add them to publish a post.
Nonetheless, tags help organize your content by grouping related posts together in a non-hierarchical system, unlike categories. Moreover, tags are typically more topic-specific and unique than categories are.
For instance, a post about a movie may be under the Movie category and have specific tags for the movie title, producer, writer, genre, actors, etc.
You can add and manage tags from your WordPress dashboard in Posts > Tags. When editing a post, you can also add tags by clicking on the gear icon at the top-right of the screen and then the Tags section.