A HI region (read as "H one region") is a cloud of generally neutral atomic hydrogen, typically with on the order of 1 in 10,000 atoms ionized. Their typical temperature range is ~100 K, in which case the material is referred to as a cold neutral medium (CNM). Above that, they are unstable, but are stable again above 10,000 K. Between those temperatures, they generally cool or heat to a stable state. They typically also have some helium and other elements. They are not very luminous, mostly the 21-cm line, which along with the spectral lines of other elements embedded in the cloud, allows them to be detected.
Other kinds of hydrogen clouds include HII regions and molecular clouds. HI regions and HII regions are termed diffuse clouds and molecular clouds (which are even cooler and more dense) are termed dense clouds.