#include means to include the later things, and iostream is the abbreviation of input output stream, which means iostream. So # contains

The iostream library is based on two types named istream and ostream, which represent input stream and output stream respectively. A stream is a sequence of characters read or written from an IO device. The term "stream" attempts to explain that characters are generated or consumed in chronological order.

The standard library defines four IO objects. When processing input, use an istream type object named cin. This object is also called standard input. When processing the output, use an ostream type object named cout (pronounced see-out), which is also called standard output. The standard library also defines two other ostream objects named cerr and clog (pronounced "see-err" and "see-log" respectively). Cerr objects, also known as standard errors, are usually used to output warnings and error messages to program users. And the clog object is used to generate general information about program execution? [ 1]? .

References:

Iostream- Baidu encyclopedia