Lots of people in the public education system speak about rubrics a lot. I used to wonder if "rubrics" were a code word or something akin to a secret handshake. But it's quite simple: rubrics are helpful to explain expectations.
When my kids were growing up I always left them a list of jobs to do while I was away. As an experiment, I tried leaving a list with three different levels. For the minimum level, there was no money, it was a part of their regular repsonsibilities. For the second level, there were additional jobs and additional pay. If they really wanted to earn some money, they could work to the third level of jobs.
In a classroom, rubrics explain the expectation level for students to earn different grades on an assignment. Through a rubric, the teacher describes what needs to be included to get the best grade and how that grade will be evaluated. For example, one bullet in the rubric could be, "a bibliography with at least 5 sources."
The best thing about using rubrics is the additional information and a clear description of what it means to be in level one, two or three. Rubrics are meant to eliminate the guessing.