While left shift operator (<<) using before std::cout , what does it work for?

Have got compiler error like this below:

file.C:2917:37: error: no match for 'operator<<' (operand types are 'std::basic_ostream<char>' and 'std::basic_ostream<char>')
 std::cout << "observerIndex : " <<
                                 ^

Code lines :

std::cout << "observerIndex : " **<<**
std::cout << pobserverIndex -> observerInt() ;

Could anyone please tell me what left shift operator(<<) is doing on there (before std::cout << pobserverIndex -> observerInt())?