string problem

[lulipeng]

Now i am learning <<c++ primer>>,i ran into a problem.
Please do me a favor.

question:
Operators return left operand as results.So we can put some inputs or ouputs
together:

string s1,s2;
cin>>s1>>s2;      //read first input into s1,second into s2
cout<<s1<<s2<<endl;   //write both strings 

can you explain what that means?thank you