You can think of a tag as a keyword for the content of a work. When you make a post, you need to include at least one tag. It's even more fun to search for works that use the same tags as your posts.
Furthermore, it's a lot easier to look for works on pixiv when the appropriate tags are attached.
Please note, however, that it can take a bit of time for a work to show up in the search results for a certain tag after you post it.
Works can have up to 10 tags.
In addition to the creator, other users on pixiv can attach and remove tags.
If the creator doesn't want other users to edit the tags on their works, they can lock that work's tags.
* The creator can do this at the time of posting or by editing their work and checking the box, "Don't allow other users to edit tags".
Only the creator can remove tags they've personally added to their work. Other users' tags can be removed by anyone (unless the creator locked their tags).
・History of deleted tags
You can press the + button to open a work's tag history. This will display previously deleted tags, which you can edit or restore.
It's useful when an appropriate tag was accidentally deleted.