idea
Because of the Wave-particle duality, we can't measure at the same time and with precision the position of an electron and its energy level (momentum).
This is outlined by the equation:
$$ \delta_x \times \delta_p \ge \frac{h}{4\pi} $$
With $$\delta_p$$ the stddev for momentum and $$\delta_x$$ the stddev for position.
If $$\delta_p$$ drops, as observation of momentum is getting more precise, then $\delta_x$$ increases. And vice-versa.
It is different from the observer effect in that it's unrelated to observation impacting the behavior of the observed system. Instead, it is a fundamental property of quantum systems that the combination of their position and momentum are probabilistic rather than deterministic.