I assume you mean the header <iostream>. iostream is a header or library that contains basic input/output (i/o, io) functions. Input/output functions are basic tasks by computers, which literally are actions that take something and give back something else. A stream is a sequence of data that sends information in and out of the program. You can liken this to a real stream, where the water only can flow one way. Input and output are different streams that lead in opposing directions.
