Quantum mechanics tells us that when we measure an observable quantity of a system, we only ever see certain values -- the eigenvalues of a Quantum Operator, and that afterwards, the system will behave as if it were in the eigenstate corresponding to that eigenvalue.
It turns out that at least in simple systems, the process of repeatedly measuring a value can inhibit the transition of that system to any other state. In effect, every measurement resets the clock for a transition. To see this effect, we need only to make our measurements much faster than the typical time for a transition to occur.
This works very well for simple systems such as an electron spin, or photon polarization. However, for more unstable and complicated systems like a radioactive nucleus, the energy pertubations added by such repeated measurements can have the opposite effect, called the anti-zeno effect, and greatly speed up the decay.