Heisenberg's Uncertainty principle states that both the location (or position) and momentum of a particle can not be simultaneously ascertained accurately. The more accurately we try to measure one of these quantities, the less accuracy we will achieve in the measurement of the second quantity. The principle can be mathematically stated as:
`Deltax Deltap > h/2pi`
where, `Deltax` is the uncertainty in the measurement of position , `Deltap` is the uncertainty in the measurement of momentum and h is the Planck's Constant.
This is of the principal concepts of quantum mechanics. Since the value of Planck's constant is small (`6.626 xx 10^(-34)`), this principle has meaning only for very very small particles. At large scales, the uncertainty in measurements would be relatively insignificant, and unlikely to be even noticed.
Hope this helps.