A case report is a brief description of a specific clinical case that has unique features. They may reveal a new disease, a new intervention, a new reaction to treatment, an unreported complication, a new use of a tool ... something novel that happened during a clinical case.
They may stimulate further research on a topic.
They are considered a good introduction to research because it allows students to practice their writing, be introduced to academic publishing and the peer review process, and strengthen relationships with clinical faculty.