A rubric within Google Classroom is a type of grading form which consists of a set of criteria, each have several descriptive levels, with a numerical grade assigned to it. Rubrics have been used for years in coursework mark schemes as they offer the opportunity to holistically mark work based on the premise of “best fit.”
I have quickly put together a couple of assignments based on GCSE questions for History, English and Maths. I've certainly not done as good a job of translating the mark schemes into rubrics as a specialist would, but I hope there's enough to give some food for thought.
To see the rubrics from the student's side, feel free to join the Classroom using this link. If you want to submit some 'work', you certainly won't be judged on your knowledge of History by me!!
I have also put a small amount of bogus 'work' in there so that I can demonstrate how work is marked. If you want to join as a teacher to see what the marking process looks like, you'll need to give me a shout. - Tim McLardy
Phil Miller has been good enough to put together this comprehensive guide to using rubrics on Classroom, which walks you through making a rubric, having students complete the work, marking the work and returning it for further improvement.