What is the difference between $message and $$message ?

What is the difference between $message and $$message

The main difference between the $message and $$message is given below:

$message$$message
$message is a regular variable.

$$message is a reference variable.

It has a fixed name and stores a fixed value.
It stores data about the variable.

Data stored in $message is fixed.

The value of the $$message can change dynamically as the value of the variable changes.