Edit
When the skin on the forehead (the brow) becomes creased due to concentration or anxiety. Very commonly these skin wrinkles aren't explicitly drawn but are depicted as eyebrows with inner tips bending up.
Raised eyebrows may skip this tag if the entire eyebrow is a slight curve.
Examples
- Danbooru post #5295546: raised
- Danbooru post #5306035: horizontal
- Danbooru post #5303894: v-shaped
See also
1

Pixiv
Twitter
Bluesky
Weibo
Lofter
Tumblr