A rubric is a scoring tool that lists the criteria for a piece of work. In other words, the rubric tells students what “counts” for their grade.
Rubrics are particularly useful for assignments where students have to do something like write an essay, give a speech, or perform an experiment–performance tasks. Performance tasks are often somewhat complex, so educators agree that it’s a good idea to help students understand exactly what the teacher expects them to do–before starting the assignment–via grading standards. Rubrics are a prime example of this idea.
A true rubric also communicates gradations of quality for each criteria; it helps students distinguish between “excellent,” “acceptable,” or “poor” performance, for example.
Technically speaking, Performance Gradebook does not generate rubrics. Rather, it generates rubric-based reports for students. Their reports include a list of the criteria you set for an assignment (i.e., the specific things you wanted them to do) + information on how they performed for each criteria. You add the names for the performance indicators (“excellent,” “acceptable,” etc.) when you set up your assignment.