First of all, the phase of a material (whether it is gas, liquid, or solid) depends strongly on both its temperature and pressure. For most liquids, applying pressure raises the temperature at which the liquid freezes to solid. A solid is formed when the loose, meandering molecules of a liquid get slow enough and close enough to form stable bonds that pin them in place. When we apply pressure to a liquid, we force the molecules to get closer together. They can therefore form stable bonds and become a solid even if they have a higher temperature than the freezing point at standard pressure. Water is somewhat unique, though. Water molecules spread out when they are bonding into a solid crystalline structure. This spreading-out action leads ice to be less dense than liquid water, causing ice to float. This spreading-out action of the water molecules during freezing also means that applying pressure to water lowers the freezing point. If you apply enough pressure (making it hard for the water molecules to spread out into the solid structure), you can have liquid water several degrees below zero degrees Celsius.