Access Project Settings > Gameplay Tags

From this screen you can add new tag files, mark them as “registered” and add or edit the tags within.
Add / Edit
To modify a file you need to first select it

Click the desired file and a green tick mark will appear to its side, you will note a new text box appears below the Search box … you can type the path of a new tag and press “Add Tag” to add it to the selected file.
To edit a tag simply click it in the tree view.

Registration
To the right of the file name is a button “Registered” when toggled on this file will be compiled into the game and its tags made available for set operations.
Set operations is a feature of the GameplayTagCollection and lets you perform tests like “Contains Tag”, “Is Parent” and similar. This is useful for some cases such as player stats, inventory, class/progression systems, ability systems, etc.
It is not a requirement for using tags in general however, if your tags do not require hierarchy aware set operations you can leave “Registered” unmarked to save storage. non-registered tags can still be added to GameplayTagCollecitons but will always return false when performing set operations (is parent of, contains, etc.)
