What is a triple point? What is the difference between the triple point and boiling point?

Hint: There are three phases of any substance, for instance water, can exist in solid, liquid and gaseous form. These three phases can coexist in equilibrium with one another; this point is the triple point. Boiling point is the point at which the pressure of any liquid becomes equal to atmospheric pressure and the liquid starts converting to vapor.

Complete answer:
As we know a substance can exist in three phases that are solid, liquid and gas. By changing the conditions of the temperature and pressure a gas can easily be liquefied and a liquid can be solidified. So, the conditions of temperature and pressure at which a substance can exist in all these three phases in stable thermodynamic equilibrium with one another is called a triple point of that substance.
The point at which water can exist in thermodynamic equilibrium in all the three phases is the triple point of water that is $0.01{}^\circ C$ (273.1 K) temperature and with 611.657 Pascal of partial vapor pressure.
The difference between triple point and boiling point is that, at boiling point the vapor pressure of the liquid becomes equal to atmospheric pressure and boiling point will always be higher than the triple point, it cannot be reduced below triple point.
So, triple point is the point at which all the three phases of a substance co exist in stable equilibrium.

Note:
When plotted with pressure versus temperature curve, for water, the fusion curve, sublimation curve and the vaporization curve all meet at the point and the point is called the triple point. These curves can be represented in the form of a phase diagram. While a critical point is the point at which the substance can exist in a supercritical fluid phase.