The Office UI Fabric is a Microsoft-created collection of UX frameworks you can use to build Fluent experiences that fit seamlessly into a broad range of Microsoft products. Use this tag when your question is specific to the Core of the UI Fabric.

The Office UI Fabric is a Microsoft-created collection of UX frameworks you can
The "Fabric" contains cross-platform styles, controls and resources needed to build custom apps and user interfaces that "fit" in the Microsoft Office ecosystem.

UI Fabric for the web is available in two flavors: Fabric React and Fabric Core.
Fabric Core is an open-source collection of CSS classes and Sass mixins that give you access to Fabric's colors, animations, fonts, icons and grid.

for more details, see https://developer.microsoft.com/en-us/fabric
see also