URMs are Under Represented Minorities. This refers to the context of one’s race, primarily. And the term (as it explains) denotes people of races/categories that are under represented in certain: institutes, majors, regions, etc.
In much of the US, this term applies to groups such as Blacks, Hispanics (though Hispanic is more of an ethnic term, rather than a racial term, but that’s a discussion for another thread), Native Americans, and other racial groups not represented well enough, considering their overall population relative to the country at large.
This does mean, unfortunately, that there can be ORMs, Over Represented Majorities. These are usually White and most Asian descents.