Heisenberg's Uncertainty Principle states that the more precisely one property of a particle is measured, like position, the less precisely another related property, like momentum, can be known. This is due to the wave-particle duality of matter at the quantum scale. It is impossible to simultaneously know both the exact position and momentum of a particle. The principle also applies to energy and time - the more precisely energy is known, the less precisely time can be known and vice versa. This inherent uncertainty is a fundamental aspect of quantum mechanics.