A rubric is a device used for assessing observable performance.
They can come in many forms. They can be as simple as a checklist or as complex as a five-level, multiple standards matrix of conditions. They are used for anything as simple as assessing essay questions on a test, assessing a written exam, to complicated training exercises involving multiple jurisdictions and organizations.
They are essential to assessing performance because, when created correctly, because they create an objective tool that any instructor can use and come to the same conclusions. In scenario-based training and assessment, when no rubric is used, assessment is wholly dependent on the instructors observations and subjective judgement. At best, it is inconsistent between iterations of training and instructors, at worst, it can expose biases and personal animus.