Tag is one of the pre-defined taxonomy in WordPress. Users can add tags to their WordPress posts along with categories. However, while a category may cover a broad range of topics, tags are smaller in scope and focused to specific topics. Think of them as keywords used for topics discussed in a particular post.
In WordPress, if a user does not select a category for a post, then it is automatically filed into the default category. However, tags are not automatically added to a post unless a user adds them, which makes them completely optional, but there is not limit as to how many you can add
When a visitor clicks on a tag they are taken to the archive page where all the posts with that tag are listed.