Understand immutability

Hello, I’m new to Elixir, I’m reading a book called Learning Functional Programming with Elixir I’m reading a subject of how Elixir data are immutable but when I just create a value in iex shell (Elixir (1.11.0) ) like this:

message = "Hello, World!!!"

display the above variable I get what I’m expecting but what I do not understand is, if I mutate the message variable there’s not an error.

message = "Hello!!!"

displaying again message variable I get no error and the output is Hello!!!

Please help me to understand what immutability means and how it works in Elixir