From Festipedia, hosted by the FR Heritage Group

Tags (i.e., edit tags) are brief messages that the software automatically places next to certain edits in histories, recent changes and other special pages. They can be used to detect potentially harmful edits or software bugs. Tags also are used to identify how someone made an edit, such as via mobile. The list of tags is at Special:Tags. Edit filter editors can direct an edit filter to tag edits matching the filter.

Edits may be filtered by entering a tag in the Special:Contributions or Special:RecentChanges Tag window.

Use and formatting[edit]

Tags should not be overly specific; instead the log should be directly used for review of edits.

For each tag, the respective appearance and description system messages can be accessed from Special:Tags. They should be created in the usual format, using a [[Special:Tags|Tags]]: prefix for appearances and the {{tag description}} template for descriptions, with the edit filter(s) generating the tag specified.

Tags should generally not be renamed from the edit filter interface when they have already been applied to edits, as this would create a new tag. So make sure to use the correct name from the start, or change the tag description itself. Currently, there's no way to delete or reassign tags.