From the Ruby_Newbie Guide to Symbols:
Author is trying to show a simplified implementation of the attr_writer method.
#!/usr/bin/env ruby def make_me_a
It's a multi line string. The code evaluates code embedded inside string. More on multi line strings:
http://blog.jayfields.com/2006/12/ruby-multiline-strings-here-doc-or.html
P.S. Using eval is not recommended, alternatives - yield, instance_eval, class_eval.