-I Flag in GCC ( Linux )

Question

I Have found a source file bundle with a Makefile, I went through it, and In CFLAG Variable, There is a FLAG -I , I have searched on the web, But couldn't find what it actually does. Is it something relevent to the library files included in the C file? ( stdio.h,unistd.h, pthread.h )

Please point me to a source or explain in brief, What does the Flag -I does?

-Regards