John Maclean asked:
> Every language and profession has it's own language. Ruby is no
> exception. Can someone explain to me what the two terms above mean and
> why they are so crucial to Ruby?
>
> Are they methods/classes ( don't think so) /modules/some other thing?

Immutable is not a Ruby-specific term. Its meaning in this group is pretty 
much its dictionary definition, that is, "not subject to change." We (and 
other OO programmers) apply it to objects that don't have "mutator" methods, 
methods that change the object itself.

Say you have an object, and offer it to a function of some kind, then check 
its value afterwards. If the object is immutable (like a Fixnum in Ruby) 
your variable is guaranteed to be the same as before. Otherwise, that 
function may have altered the object.

The method Object#freeze lets you make any object immutable.

Enumerable (in Ruby) is a module, included by Array and Hash. For 
documentation:
http://www.ruby-doc.org/core/classes/Enumerable.html

From that page:
" The Enumerable mixin provides collection classes with several traversal 
and searching methods, and with the ability to sort. The class must provide 
a method each, which yields successive members of the collection. If 
Enumerable#max, min, or sort is used, the objects in the collection must 
also implement a meaningful <=> operator, as these methods rely on an 
ordering between members of the collection."

Cheers,
Dave