Header files allow programmers to separate certain elements of a program’s source code into reusable files. Header files commonly contain forward declarations of classes, subroutines, variables, and other identifiers. Programmers who wish to declare standardized identifiers in more than one source file can place such identifiers in a single header file, which other code can then include whenever the header contents are required. This is to keep the interface in the header separate from the implementation. The C standard library and C++ standard library traditionally declare their standard functions in header files.
exm of Header files:
Header files provide the definations and declerations for the library functions.
So each header file contains the library functionss along with the necessary definations and declerations.