Stories can be tagged to help your organize your library. These tags don't change anything about your stories when they're published1. They're only visible in Twine itself.
A tag always has a name, but it can also have a color if you like from a set of
predetermined ones. Tag colors are only used to help distinguish tags from each
other visually. It's not possible to create custom tag colors, and a particular
tag can only have one color. That is, if you put a red
my-tag tag on a story,
you can't make that tag be green on a different story2.
Another limitation of tags is that their names are not allowed to contain
spaces. If you try to enter a space for a tag name, then Twine will convert it
to a hyphen for you (i.e.
my tag becomes
When a tag is added to your story, it will displayed on the story card. It looks like a small sticker.
Once tags are added to your stories, you can filter the Story Library screen so that only stories with certain tags are visible.
Select a story, then choose Tag from the Story top toolbar tab. You can either use an existing tag or create a new one.
Select the tag sticker that's on a story card, then choose Remove.
Choose Story Tags from the Library top toolbar tab. This will open a dialog showing all tags used in your library. Use the Rename button next to a tag to change it. This will affect all stories in your library with the tag.
Select a tag sticker on a story card, then choose the color name from the menu that appears. This will change the color of this tag on all stories in your library. You can also change it using the Story Tags dialog as explained in the Renaming a Tag section above.
Story tags are available to story formats, so it's possible that one might change its behavior based on tags applied to stories. But this is discouraged so that authors can freely use tags as they like.