In quantum mechanics, one hears much talk about ** operators**. The Wikipedia page for operators (a good page to know for those interested in QM) first has a section about operators in classical mechanics. The larger quantum section begins by saying:

*“The mathematical formulation of quantum mechanics (QM) is built upon the concept of an operator.”*

Operators represent the *observables* of a quantum system. All measurable properties are represented mathematically by an operator.

But they’re a bit difficult to explain with plain words.