It means whenever you change the value of String, you create a new object and make that variable refer to new object.Appending a string is also a same kind of operation internally.
Each time you use "+" operator or ( String.concat() ) , a new string is created, old data is copied and new data is appended. Old string will be garbage collected.
When to use "+":
1). Multiline Strings
String text= "line a\n"+ "line l\n"+ "line b";
2) Short messages
When you have multiple string blocks to concat use StringBuilder or
StringBuffer( if multithreading is used ) class.