what is plank constant?

Planck's constant (denoted h) is a physical constant that is used to describe the sizes of quanta. It plays a central role in the theory of quantum mechanics, and is named after Max Planck, one of the founders of quantum theory. 
Heisenberg showed that the uncertainty in the position of the particle times the uncertainty in its velocity times the the mass of the particle can never be smaller than a certain quantity,which is known as Planck's constant.This limit doesn't depend on the type of particle,or on the way in which one tries to measure the position or velocity of the particle.
Planck's constant is also used in measuring energy emitted by light photons, such as in the equation E=hv, where E is energy, h is Planck's constant, and v (Greek letter nu) is frequency.
The value of Planck's constant is:
6.626 x 10^-34 J-second