Output Chaining

cout << "Hello World!" << endl;

Generally, since << is left-associative,

return X <<(ostream X, values)

... the left referenced ostream is returned and the next values in the chain added to it.

Consider

cout << b << c << endl;
(cout << b) << c << end;

Stream (buffer) contains b: (cout << c) << endl;

Stream (buffer) contains b, c cout << endl;

The manipulator endl writes the stream out from the memory buffer to the output stream.