A rating scale is a scale against which a learner's performance is rated, producing a quantifiable result. The rater is usually trained in applying the scale before they rate real samples of language.
The IELTS exam is an example of an exam that uses a rating scale.
In the classroom
Developing - or adapting- a good rating scale can improve evaluation of learners by providing clear and consistent criteria. These can also be communicated to the learners themselves and form part of a syllabus structure or portfolio. Learners can rate their own, or a classmate's work against a rating scale to help them become more aware of what is important to work on.