In general, when a student is in the process of learning new material, she understands some of it, but not all.  The specific work of learning takes place at the boundary between the domains of what she knows and what she doesn't.  One of the most important purposes individual student work can serve is to shine a light on that perimeter. Only the student can know that boundary intimately, which is one of the reasons why self-directed learning is generally more effective than teacher-directed learning.  

If a student is working on autopilot, unaware of what she does and does not understand, she will not learn very well.  However, students can be taught to be self-aware while working. Well-designed work teaches a student how to pay attention to how well she has mastered the material and how to steer the learning process accordingly.

One way students’ metacognitive skills can be cultivated is through self-evaluation.  When a student assesses the level of her understanding of her work, she is forced to pay more careful attention to the material.  She needs to stop and ask “Do I get this?” before moving on. She is also clarifying whether she needs to do further work to master the material.  This serves as the basis for deciding what to do next and enhances self-directedness.

It is critical that students learn to be realistic in their self-evaluations.  Without a clear and accurate appraisal of her level of understanding, a student will be much less likely to make good choices.  The accuracy and honesty of self-assessments need to be checked regularly when students are first learning this skill, and periodically thereafter.  The intent is not to “catch” students being dishonest in their self-evaluation, but rather to help them become more clear-headed and subtle in their analysis.

The more specifically a student can identify what she has not yet mastered, the more effective her learning becomes.  Zeroing in on precisely the concept or skill that is still eluding her leads to more accurate and useful questions in the follow-up conversation.  Those questions drive the learning process and make the discussion more productive for everyone.