Final is a convention that comes from Java. Final variables can contain any value, but once assigned, a final variable can't be reassigned to any other value. So if I write final x = 10; and then try to write x = 11; then it will throw an error because I am trying to resign the variable x. final is also used for instance variables in an object, this means that if you use final for a property inside of an object and pass that property to the constructor then the property will not have an implicit setter method created for it.

class SomeClass {
  final int a, b;
  SomeClass(this.a, this.b);
}

void main() {
  var sc = SomeClass(10, 20);
  print(sc.aa); //prints out 10
  sc.a = 30; //Warning, no such setter method for a or b.
}

It is kind of important to keep in mind that the final keyword does not make the variable's object immutable.

class Person {
  String name;
  int age;
  Person(this.name, this.age);
}

void main() {
  final person = Person("John", 52);
  person.age = 32; //this is valid even though the person variable is final
  print(person.age); //prints out 32
}

If you want to play around with some dart to check to see how things work, you can use the dartpad website though I am pretty sure DartPad still uses Dart 1 syntax this means you would need to add new keywords to some of the pieces of code above.