'# include' is a PREPROCESSOR DIRECTIVE, identified by the '#' symbol. The C preprocessor carries out text-manipulation directives as a preliminary step to compilation. This one directs the preprocessor to "paste" the text from the indicated file into this file before compilation. The #included files oops.h and stdio.h are called HEADER files, and contain C source code declaring constants, data structures, and library functions which we will use in this program.
The C language is very terse, but is readily extended by the use of headers and function libraries.