听课 means 'to attend a class or lecture', often with the nuance of sitting in to observe rather than actively participating. It is a separable verb (离合词): the verb 听 and the object 课 can split when adding measure words, duration, or other elements, e.g., 听了一节课 ('listened to one class session'). Unlike 上课 (to have a class), 听课 focuses on the act of listening as a participant or auditor.