Heisenberg's uncertainty principle tells us that it is impossible to simultaneously measure the position and momentum of a particle with infinite precision.
In our everyday lives we virtually never come up against this limit, hence why it seems peculiar. In this experiment a laser is shone through a narrow slit onto a screen.
As the slit is made narrower, the spot on the screen also becomes narrower. But at a certain point, the spot starts becoming wider.
This is because the photons of light have been so localised at the slit that their horizontal momentum must become less well defined in order to satisfy Heisenberg's uncertainty principle.