friend istream & operator >>(istream &, vector &);

Its a operator overloading code. which should follow the genric syntax like
"returntype class operator op(argu)" to declare the function..

Here istream is class and >> op....arguments going to have referal type....
Question is what "&" signifies, the one is before operator
is it going to retrun in referalt type

It means that >> will return a reference to its base class, in this case istream. This allows the programmer to 'chain' >> together:

cin >> iTest >> cTest >> iTest2; etc.