The skill meter on an individual student report is based on an algorithm that takes into account the difficulty level of each question answered, as well as how the student performed on previous attempts at a given practice, and thus does not map to a 'percent correct' calculation.
Beginning - new to concept (Red)
Just being introduced to the concept for the first time. Will not be able to apply core skills from this concept to future concepts without additional learning. CK-12 recommends taking advantages of the variety of learning modalities offered to gain an understanding of this concept.
Exploring - starting to understand (Orange)
Has begun to demonstrate initial grasp of the concept but does not yet display understanding of the core skills/topics. CK-12 recommends continued learning on this concept before moving on to concepts that use this as a prerequisite.
Developing - demonstrating familiarity (Yellow)
Approaching a minimal applicable level of understanding of the concept. Able to state some key points or ideas for this topic. CK-12 encourages exploration of new modalities or review of a favorite modality to gain a deeper understanding of this concept.
Proficient - understands core concept (Light Green)
Solid understanding of the concept. Able to apply material to subsequent topics with prompting. Able to describe primary and support ideas in alternative wording. CK-12 encourages continued learning by exploring applications of this concept or persuing related concepts.
Mastery - deep and demonstrated understanding (Dark Green)
Deep understanding of the concept. Beyond grasping the core skills/content areas, can answer advanced questions and apply the material to new situations and other areas. CK-12 recommends progression to the next concept and/or application of this concept through various projects to expand learning.