Hydrosol AKA floral water AKA essential water is a product of the steam distillation of plant material. Steam rises through the plant material, opening up its glands, bursting its cells, and the oils and essence of the plant are carried up and away by the steam. The steam, trapped in a distilling arm, travels to a condenser where cool water is ran through it and there the steam condenses. The water droplets that form in the condenser contain the essential oils of the plant matter, as well as the organic acids and any water soluble components of the plant. Because water is heavier than (most) oils, the water sinks to the bottom of the receiver and the oil floats on top. The water that is collected is the hydrosol. It is fragrant, though it does smell differently than the plant material, but it still contains the essence of the plant. It also contains a teeny tiny fraction of essential oil.
You can use hydrosol in the same manner in which you would use essential oil. In some cases, essential oil is too potent to use directly on your skin, whereas hydrosols, unless you’re allergic to the plant the hydrosol came from, are perfectly fine to use on your skin. Hydrosols make wonderful astringents and toners for your face, as well as disinfectants for scrapes and burns. They can also be used as household cleansers and as insect repellents.