what does it mean by delcaring a field as final code example

Example: what it means when create final variable in java

First of all, final is a non-access modifier applicable only to 
a variable, a method or a class

When a variable is declared with final keyword, its value can’t be modified, 
essentially, a constant. This also means that you must initialize a 
final variable. If the final variable is a reference, this means that 
the variable cannot be re-bound to reference another object, but internal 
state of the object pointed by that reference variable can be changed 
i.e. you can add or remove elements from final array or final collection. 
  It is good practice to represent final variables in all uppercase, using 
  underscore to separate words.