The tag boxes let you filter the story list. Each tag supports three states that you cycle by clicking:
Neutral (plain) — no effect from this tag.
Include (blue/highlighted) — only stories that have this tag are shown. Multiple includes are ANDed.
Exclude / "not" (red) — any story that has this tag is removed from the list.
Click a tag once for include, again for exclude, again to return to neutral. You can combine includes and excludes.
On the upload form, click tags to select the ones that apply to your story (at least one required).
Click again to deselect. Highlighted tags will be attached to the story.
Tag Legend (short label → description)
(This list is also shown in hover titles on the tag pills.)