Strings In Java - Adding Characters

It works when i do this ,
 
String a = "*";
for(int i = 0; i++; i<10){
 
System.out.println(a);
a = a +"*";
}

but it doesn't work the other way,

String a = "***************";
for(int i = 0; i++; i<10){
 
System.out.println(a);
a = a - "*";
}

I searched about this online and read strings are unmutable in java but what does that exactly mean ? If the string cannot be changed how i am able to add the characters but not remove them.