String is immutable. What exactly is the meaning? [duplicate]

Answer 1

String S1="abc";
S1.concat("xyz");
System.out.println("S1 is", + S1);
String S2=S1.concat("def");
System.out.println("S2 is", + S2);

This shows that once a string object is create it cannot be changed. EveryTime you need to create new and put in another String. S