The observer effect refers to situations in science where the act of observing a system has an impact on the system being observed. Checking pressure in a tire, for example, is done by causing a release of air which, in turn, causes a slight change in the tire pressure. This is a classic example. Scientists (especially psychologists) have to be especially careful when planning their research methods to avoid interfering with the results they intend to get.
In the realm of quantum physics, there's a more powerful form of the observer effect, as exhibited by the Schroedinger's Cat thought experiment and the quantum double slit experiment. It appears that in cases like these, the very act of making the measurement doesn't just modify the system, but actually results in fundamentally different physical behaviors within the system.