Dashicons, the WordPress admin icon font. For the official documentation, please refer to the WordPress Developer Resource.
The Dashicons project is no longer accepting icon requests. Here's why: Next steps for Dashicons.
For any bugs that appear within WordPress Core, please create a new ticket on WordPress Trac. Use the "Administration" component and the "ui" focus when creating the new ticket, and be sure to include "Dashicons" somewhere in the text of the ticket.
To build Dashicons, make sure you have Node JS installed. Then type npm run build on the commandline to generate the minified SVG files and sprite.
Once you're installed, to add an icon, save an SVG in the sources/svg folder, then run the npm run build command on the commandline.
If you want to add your icon to the Dashicons library, open a new Pull Request here with the new SVG icon, and wait a review.
If you're adding an icon for Gutenberg specifically, add it inside the sources/svg/gutenberg folder. It will be processed for everything but the font files.
Dashicons is licensed under GPLv2, or any later version with font exception.