Water changes from solid to liquid at 32 F degrees and changes from liquid to gas at 212 F degrees at sea level air pressures. Temperature is the measure of relative motion of molecules. Below 32 F the water molecules are locked into restricted motion within the solid ice crystals. Between 32 F and 212 F the more agitated motion caused the molecules to move around in the liquid. So boiling is the generation of bubbles of gas in the heated liquid.
Pressure affects the boiling point of a liquid.
If pressure is reduced, the boiling point is reduced. Food placed in a vacuum chamber can be freeze dried because of this effect. Pressure cookers raise the boiling point of water to help us cook foods faster. You can learn more about heat and properties of materials in physics courses and general physics books.