In programming, a header refers to a file or section of code, typically placed at the beginning of a source file, that contains declarations, definitions, or metadata used by the program. Headers often include information such as function prototypes, macros, constants, and data structure definitions. They help organize code, promote reusability, and enable easier collaboration between different parts of a program or among multiple programmers. In cybersecurity, understanding headers is important because they can affect how software components interact and may reveal vulnerabilities if not properly managed or secured.