Where is the file iostream.h?
This file is located somewhere in your include path. The include path indicates the directories on your computer in which to search for a file, if the file is not located in the current directory.

Why do I need to include iostream.h?
In this case,
iostream.h is a file containing code for input/output operations. You need to include iostream.h so that the compiler knows about the word cout, which appears a couple of lines below.