The in-degree of a node is the number of edges pointing to it. For courses, in-degree means "how many prerequisites are left". When in-degree becomes , the course is ready to take. You have finished all its prerequisites. Kahn's algorithm exploits this: start with all courses that have in-degree (no prerequisites).
Take them, then remove their outgoing edges. This decreases the in-degree of their neighbors. When a neighbor's in-degree drops to , it becomes ready. You repeat until all courses are taken or you discover a cycle.