Writes a small introduction

doc/Language/iterating.pod6

@@ -6,7 +6,9 @@
 
 =head1 The C<Iterator> and C<Iterable> roles
 
-TBD
+Perl 6 is a functional language, but functions need something to hold on to when working on complex data structures. In particular, they need an uniform interface that can be applied to all of them. This interface is provided by the L<Iterator> and L<Iterable> roles.
+
+The C<Iterable> role is quite simple. It provides the stub for a C<iterator> method, which is the one actually used in most ocassions. 
 
 =head1 How to iterate: contextualizing and topic variables