Source code comments are usually treated in software engineering as documentation artifacts that support readability, maintainability, and long-term comprehension. In programming education, however, comments may also function as pedagogical scaffolds by helping learners externalize reasoning, clarify intent, and reflect on code. This article presents a PRISMA-informed qualitative systematic review with layered evidence on the pedagogical role of source code comments in programming education. Searches in Scopus and ERIC produced 50 unique records; 36 were assessed in detail and resolved into 18 primary synthesis studies, 7 supporting/contextual studies, and 11 advanced-stage exclusions. Because the evidence base is heterogeneous, the review uses qualitative layered synthesis rather than meta-analysis. The findings suggest that comments are best understood pedagogically as explanation-centered learning supports, especially for code comprehension, self-explanation, debugging and reflective reasoning, and formative assessment of student thinking. Direct comment-focused evidence remains limited and concentrated mainly in highereducation and novice programming contexts; adjacent explanation-centered studies clarify plausible mechanisms but do not by themselves establish comment-specific effects. The review concludes that comments can become pedagogically meaningful when deliberately integrated as scaffolds for explanation, comprehension, and reflection.