Heisenberg realized that one implication of quantum physics is that the act of measurement always disturbs the object measured. The Uncertainty Principle applies to all objects, but is only significant at the atomic or subatomic level. At such scales, there are discernible limits to how certain we can be about an object's position.
Light consists of discrete units, or quanta, of energy known as photons.
Shining a light on an electron means bombarding it with photons, each of which has a big effect on the electron. Heisenberg and his fellow quantum pioneers recognized that very energetic photons will give a more accurate reading of the electron's position, but they are also more disruptive. Let's say we could measure both the position and velocity of every sub-atomic particle with infinite accuracy. Then we could measure the position and velocity of every particle in your body, and predict the future positions and velocities of every particle in your body. The Uncertainty Principle is the physical reason why free will is possible.