A rubric is a guideline for rating student performance.  Most band, choir, or orchestra festivals have such guides for adjudicators when they rate ensemble performance.  The guidelines specify what a performance is like at various levels (superior, excellent, good, poor) and, usually, on various musical attributes (tone, intonation, balance, technique, etc.).  The key elements of a rubric are the descriptors for what a performance is like within the full range of possible performance levels.