Simply put, Heisenberg's Uncertainty Principle states that it is impossible to know both the exact position and the exact velocity of an object at the same time. However, the effect is tiny and so is only noticeable on a subatomic scale.
Werner Heisenberg (1901-1976) was a German physicist who helped to formulate quantum mechanics at the beginning of the 20th century. He first presented the Heisenberg Uncertainty Principle in February 1927 in a letter to Wolfgang Pauli, then published it later that year.
Light can be considered as being made up of packets of energy called photons. To measure the position and velocity of any particle, you would first shine a light on it, then detect the reflection. On a macroscopic scale, the effect of photons on an object is insignificant. Unfortunately, on subatomic scales, the photons that hit the subatomic particle will cause it to move significantly, so although the position has been measured accurately, the velocity of the particle will have been altered.
By learning the position, you have rendered any information you previously had on the velocity useless. In other words, the observer affects the observed.