This a neat feature that can greatly speed up your classes at runtime. However, it can incur some costs as it tells Moose that you will not be making any more changes to that class. This then allows Moose to generate code specific to your class, like an inline constructor which makes object construction much faster.
# Make a class immutable by calling make_immutable on your class's metaclass object:
__PACKAGE__->meta->make_immutable;