a preprocessor which process some other files before our 
main program.
eg:#include<stdio.h>

Preprocessor is a parser which parses the code before 
compilation.It describes path of compilation,includes the 
definitions of standard C functions and subustuites the 
variables depending on the pre processor directive 
used.Also can be used to output the comments and error 
messages during compilation.

preprocessor is which tells the compiler what header file 
we are going to use in our program 
#include<stdio.h>
will tell the compiler that it include header file standard 
input output

Each of the above answers is partially correct. A 
Preprocessor is a software tool. You include many 
preprocessor directives in your program.
e.g. include directives , #define, #ifdef , #pragma for 
conditional compilation. All these things are taken care by 
this tool. It comes into picture before compiler.

Preprocessor is the program which processes the source code before it goes to the  compiler.

In computer science, a preprocessor is a program that processes its input data to produce output that is used as input to another program. The output is said to be a preprocessed form of the input data, which is often used by some subsequent programs like compilers.