Many C programs have #include <file> statements. This directory contains the files referenced by those statements.
Many of the files contain information on data structure for reading specific UNIX files, or for getting particular information from the operating system (such as who is logged on, what is the type of a specific file, etc.). Other files simply assign a name to an integer value: an example is the EOF string, defined as (-1) in file /usr/include/stdio.h.