What is triple point of water?

The triple point of any substance is that temperature and pressure at which the material can coexist in all three phases (solid, liquid and gas) in equilibrium.Triple point of water is a single point in P-T phase diagram of water where the three phases of water coexist.

The triple point of water is defined to take place at 273.16 K, where K is the SI unit Kelvin. Unlike Celsius and Fahrenheit scales, Kelvin is not measured using degrees; we merely say "Kelvin."
It is important to note that the pressure the triple point exists at is 4.58 torr in SI units or .006 atm.

What is Heavy water?

Normal water is made of two Hydrogen atoms and one Oxygen atom. Hydrogen atom and Oxygen atoms are covalently bonded together. Normal water is designated by H₂O.

Heavy water also has same molecular structure but Hydrogen atoms are replaced by isotopes of Hydrogen called 'Deuterium'. Standard hydrogen has one Proton in its nucleus (Relative Molecular Mass = 1), Deuterium has one Proton and  one Neutron in it nucleus, (Relative Molecular Mass = 2). Hence it is 'heavier' than normal hydrogen. This leads to the water being heavier.