Hash, short for hashish, is a cannabis concentrate made by isolating the trichomes from the cannabis plant. Hash can go through various forms of processing after trichome separation to further tailor its consistency and usability. 
Because this substance is made out of only trichomes—the part of the cannabis plant containing cannabinoids and terpenes—it is much more potent than regular cannabis flowers. 
It is also more full spectrum than solvent based cannabis concentrates (such as BHO/PHO or distillate), allowing for more broad ranging therapeutic effects and a full bodied high that can be explained through the entourage effect. 
Solventless concentrates are able to maintain the integrity of their terpene and cannabinoid profile relatively well through the extraction process compared to other solvent extracted concentrates.