What is the relevance of -flag-?Infact ,what are flags?

Question

While using the functon setprecision to precise the value after decimal point in any floating point number,we need to first insert a flag....
e.g.-. float a=2.3456;
cout.setf(ios::fixed);
cout<<setprecision(2)<<a;
Will yield:2.34
We have other flags too ...like left,right,showpos...