What is difference between mutable and immutable String in java

Question

As far as I know,

An immutable string cannot be modified, while changeable strings can.

Here, I want to modify the String's value as follows:

String str="hi";
str=str+" hello";

another option is,

StringBuffer str= new StringBuffer("hi");
str.append("hello");

I'm attempting to change the value of str in both situations. Can somebody explain the differences between the two cases and provide a clear illustration of changeable and immutable objects?