In response, they developed a new kind of physics. In an astonishing burst of creativity, they kept key principles from the classical world — like the conservation of energy — but added new rules. One of these was the Uncertainty Principle, which essentially told us that reality is fuzzy at its root level. To be exact, certain pairs of properties — like motion and position — can never be known exactly. The Uncertainty Principle isn't saying there's something wrong with our instruments. Instead, it tells us there's something wrong with our classical intuitions. In particular, when it comes to motion, it tells us it's impossible to know the position and the motion of a particle exactly. The more you lock in the position of a particle, the wider the range of velocities the particle can have.
So what does this have to do with temperature?
Absolute zero should mean bringing particles to a halt. But that would imply you knew exactly where they were. You had them perfectly "localized." If that's the case, then the Uncertainty Principle demands there must be some uncertainty in their motion. They can't be perfectly known to be perfectly at rest. The deeper meaning of this this quantum logic is that the universe can never be at rest. There is a "floor" to how much things can be slowed down (or cooled). It's impossible to go below that floor (though scientists do get ever more clever in skirting its edges).
The implications of this can get pretty strange. Imagine we put a particle, like an electron, in a box. Now we ask: What's the lowest energy state of the electron + box system? In classical physics, it would just be the electron sitting there unmoving — i.e. zero motion, hence zero energy. But quantum physics won't allow such a thing as zero energy (because of the Uncertainty Principle). Instead, the system has non-zero "ground state" energy with the electron bouncing back and forth between the box walls. That's as low as you can go. The electron can't be stopped.