An operational definition is one that tells you how to demonstrate that which is to be defined. For instance, you would define gravity by dropping something. If you wanted or needed to go further you might measure its rate of fall, etc..
Measurements demand the use of operational definitions of relevant quantities. That is, a scientific quantity is described or defined by how it is measured, as opposed to some more vague, inexact or “idealized” definition. For example, electric current, measured in amperes, may be operationally defined in terms of the mass of silver deposited in a certain time on an electrode in an electrochemical device that is described in some detail. The operational definition of a thing often relies on comparisons with standards: the operational definition of “mass” ultimately relies on the use of an artifact, such as a particular kilogram of platinum-iridium kept in a laboratory in France.
Another benefit of thinking in terms of operational definitions is that it slows down your emotional reaction. And it vastly expands your understanding, both of the subject matter, and of what the speaker is really trying to sell.