this is not a bad explanation: http://en.wikipedia.org/wiki/Quantum_Zeno_effect
(Wild tangent alert!)
From that Wikipedia article:
The quantum Zeno effect takes its name from Zeno's arrow paradox, which is the argument that since an arrow in flight does not move during any single instant, it couldn't possibly be moving overall.I have always thought that this is a surprisingly silly paradox. Motion is inexorably related to time, and clearly so. The fact that the paradox depends upon the restraint of "any single instant" exposes it as just a ridiculous play on words. By definition, nothing moves "during any single instant"!
In fact, the use of the word "during" in that phrase is contradictory in and of itself. If no time passes, there is no "during".