The habitable zone around a star is the distance within which liquid water can exist. This varies by atmospheric pressure, but on Earth these are temperatures between 32 degrees Fahrenheit and 212 degrees Fahrenheit, varying slightly by altitude. In Earth's solar system, scientists estimate that the habitable zone lies between Venus and Mars; the Earth is in the center of this zone.
Liquid water is essential to all life as science understands it, but the conditions under which liquid water exists are actually quite variable. There are organisms that live in high-pressure environments, such as thermal vents at the seafloor, where the water is super heated. At normal atmospheric pressure, it would boil away. So, while up to this point scientists have based the habitable zone on the presence of liquid water and the amount of solar energy received compared to Earth, they are beginning to revise this idea. Under the right conditions, planets may have liquid water at a much greater distance from their star than previously understood.
This is because, almost unique to water among liquids, high pressure not only keeps it from boiling, it also keeps it from freezing. This is because of the unusual fact that solid water is less dense than liquid water, making the liquid phase the most compact form of water.