Header (*.h) files for most libraries; includes all header files distributed with the MySQL binary distribution.
These files may be included in C program files. Note that each individual directory will also have its own *.h files, for including in its own *.c programs. The *.h files in the include directory are ones that might be included from more than one place.
For example, the mysys directory contains a C file named rijndael.c, but does not include rijndael.h. The include directory contains rijndael.h. Looking further, you'll find that rijndael.h is also included in other places: by my_aes.c and my_aes.h.
The include directory contains 55 *.h (header) files.