Hydrosols (hydrolats, floral or aromatic waters) are one of the two products of distillation. During distillation, water evaporates along with aromatic compounds from the plant material. When the vapors are cooled, the water and essential oil separate. Most of the aromatic molecules pass into the essential oil, but some pass into the distillation water.
Hydrosols have a wide range of use, and as the concentration of aromatic constituents is much lower than in essential oils, they offer a safer therapeutic option.
These two articles that explain what hydrosols are in much greater detail: