The extrafontdb package is meant to be used with the extrafont package. The extrafont package contains the code to install and use fonts, while the extrafontdb package contains the font database. (When initially installed, this package is essentially empty, except for some directories; the database itself is created by the extrafont package).
The code and the database are separated into two packages because each time that the package containing the database is re-installed, the database gets deleted. By putting the database in a separate package, it is possible to update the code package without deleting the database and having to re-import the fonts.
Re-installing this package will re-initialize the font database, and all font will need to be re-imported.
See https://github.com/wch/extrafont for more information.