Heisenberg's uncertainty principle
It is a basic postulate of quantum mechanics. It states that it is impossible simultaneously to measure the position of a particle, say an electron, an atom or a molecule, and its momentum. It places limits on these measurements such that uncertainty in position $\Delta x$ and momentum $\Delta p$ is given by $\Delta x\Delta p \ge \hbar/2$ where (hbar) $\hbar =h/2\pi$ and $h$ is the Planck constant $6.626 \times 10^{-34}~\mathrm{J~s}$. In a classical world no such restriction occurs, but this is not what is observed experimentally.

The important part is simultaneous measurement. We can measure where an atom is to whatever precision an instrument will allow, (e.g. by xray crystallography) but we cannot in the same measurement (i.e. at same time) know what its momentum is. We can measure an atom's speed v as accurately as we like and hence momentum mv. We note its position in two places a and b, say 10 m apart, and divide distance by time to cover this distance.

The uncertainty principle places limits on the measurement of momentum mv and position that allows us to know both the position and momentum an atom (particle) has at any given instance. Ask again if this is not clear