It only takes a Google search for "standard output stream" to get the answer as #1 result: A Wikipedia page explaining standard streams in computer programming. A lot faster than asking without researching.
We are usually helpful around here, but you must learn how to research things by yourself as well.
the standard output stream of a language (in c++ its ostream cout, c is printf();, batch is echo etc etc) is the most traditional, or standard way of printing text to the screen. it is called a stream because it is streaming it through printing it to the screen