This lecture covers the concept of the sizeof operator in C programming, focusing on its application with pointers and arrays. The instructor explains that sizeof returns the size of a type or expression without evaluating it, highlighting common pitfalls in its usage. Examples illustrate how sizeof can be used with different data types, such as integers and arrays, and the importance of understanding the implications of passing arrays to functions. The lecture emphasizes that an array does not retain knowledge of its size when passed as a function argument, which can lead to errors. The instructor provides cautionary examples, including potential buffer overflows caused by incorrect assumptions about array sizes. The discussion also includes the significance of using sizeof correctly to avoid common programming mistakes, ensuring that students grasp the critical aspects of memory management in C.