I don't really get what you don't understand. I was going to suggest Ruby for Rails, but if you've finished that, you should have a rather firm grasp on both Ruby and Rails. What _precisely_ dont you feel like you understand? Basic concepts like variables and flow control? Or more advanced concepts like object-oriented design and metaprogramming?
Well, let me ask you this: can you read others' code? Put _writing_
code aside for a second, pick up some Ruby, and then try to figure out
what's going on. Can't do it? OK, pick up one of your many
Ruby/Rails books and look it up. 
They should explain the mechanics
behind it and you should be able to learn things for the future that
way.
--Jeremy