our act of observing interferes with what is being observed

 This  Uncertainty Principle asserts that we cannot know the values of certain pairs of physical variables (like position and velocity, or better, momentum) with arbitrary accuracy. If we try to improve our measure of one of the two, the other becomes more inaccurate. Note that this limitation is not due to the act of observing, as it is sometimes said. Heisenberg, trying to create an image to explain the mathematics of the Uncertainty Principle, claimed that if we, say, shine light into an object to see where it is, the light itself will push it away and its position will be imprecise. That is, the act of observing interferes with what is observed.