"You are referring to the 'Uncertainty Principle,' deduced by Werner Heisenberg early in the 20th century. 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.
"The physical reason behind this uncertainty is that measurement, by its very nature, requires using some sort of energy--for example, shining a light on the object to be measured. 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. Hence, there is a tradeoff: the more precisely we know an object's position at the time of measurement, the less we know about its present whereabouts. This uncertainty cannot be eliminated by designing better instruments; it is inherent in the laws of quantum physics.
Gordon then considers the second part of the question: "One significant test of the Heisenberg principle can be thought of in philosophical terms: 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. In other words, we could predict exactly what you are going to do for the indefinite future; you would have a deterministic world which precludes free will. The Uncertainty Principle is the physical reason why free will is possible. Even with infinitely accurate instruments, we cannot predict the future actions of sub-atomic particles, and therefore we cannot predict the future of macroscopic particles (like people) either."
On a more practical level, the Uncertainty Principle explains numerous real, observed physical effects, such as quantum tunneling. Sometimes an electron will "tunnel" to a location where, according to classical physics, it could never appear. This phenomenon forms the basis of the scanning tunneling microscope, which allows scientists to map the location of individual atoms