A HI region (pronounced as "H one region") is a cloud of generally neutral atomic hydrogen, e.g., with only about 1 in 10,000 atoms ionized. Typically it is about 100K. Above that, they are unstable, but are stable again above 10,000K. They typically also have some helium and other elements. They are not very luminous, mostly the 21cm line, which along with the spectral lines of other elements embedded in the cloud, allows them to be detected.
Other kinds of hydrogen clouds are the HII region and the molecular cloud. HI regions and HII regions are termed diffuse clouds and molecular clouds are termed dense clouds.