A substance can physically exists in one of three states, solid, liquid and gas. (A fourth state plasma that can be observed is not relevant to this answer)
The temperature of a substance and the pressure applied to it determine which state the substance is in. For water, at room pressure of 100 kPa, the melting point is 0 C. The phase diagram of water shows the state it exists in at different pressures and temperatures. It is apparent that if the pressure is increased, the melting point goes up.
Water's triple point lies at the temperature 0 C. At this temperature, it is possible for water to exist as solid, liquid and gas for different values of pressure.
The melting point( liquefaction point) of a solid is the temperature at which it changes state from solid to liquid at atmospheric pressure. At the melting point the solid and liquid phase exist in equilibrium.
melting point of water at standard pressure 273.15K