Meaning of #include<stdio.h>

Stdio.h, which stands for "standard input/output header", is the header in the C standard library that contains macro definitions, constants, and declarations of functions and types used for various standard input and output operations.
Courtesy of Wikipedia..
http://en.wikipedia.org/wiki/Stdio.h

# denotes a preprocessor statement(i.e) in simpler words it's the computers way of telling that it has included the given library function in the given program.
e.g.. #include<stdio.h>
The above statement means that all functions belonging to stdio.h library is inherited to the program and all keywords relating to that is also inherited