meaning of triple point of water

DEFINITION:-

In thermodynamic, the triple point of
a substance is the temperature and pressure at which the three phases(gas,liquid and solid) of that substance coexist in thermodynamic equilibrium.

TRIPLE POINT OF WATER:-

The single combination of pressure
and temperature at which liquid water, solid ice, and water vapor can
coexist in a stable equilibrium occurs at exactly 273.16 K (0.01 °C;
32.02 °F) and a partial vapor pressure of 611.657 Pascals
(6.11657 mbar; 0.00603659 atm).At that point, it is possible to change all of the
substance to ice, water, or vapor by making arbitrarily small changes in
pressure and temperature. Even if the total pressure of a system is well above
triple point of water, provided the partial pressure of
the water vapor is 611.657 Pascal Then the system can still be brought to the triple point of
water.

one can batter understand by this phase diagram of water:-

At the triple point, water can exist in three states of matter: gas, liquid, and solid (ice). This occurs at a particular temperature and pressure of 273.16 K (.01 °C) and 611.73 pascals. The change of state can be accomplished by varying the temperature extremely slightly so as to induce the change. 

The triple point of water also represents the lowest pressure at which liquid water can exist.

And perhaps most importantly, it is where our definition of the temperature unit Kelvin comes from. One Kelvin is defined as 1/217.16 the temperature of the triple point of water. (The triple point of water is not defined by its temperature so much as it defines the measurement of temperature itself.)