First, a caveat: there is no definitive answer to this question.
Rails is more than just a framework. It's a Domain Specific Language separate from Ruby, with its own keywords, culture, and idioms. A primary function of the Rails DSL is to hide things from you. Rails handles the abstraction between a model and the database, for example, so that you don't have to write a data access model. So in this sense, Rails is simpler than Ruby–if you had to build a web application without Rails, you would have a huge amount of work to do.
On the other hand, every syntactically correct Ruby statement is a syntactically correct Rails statement. So the application logic that you write in Rails is Ruby, which is one reason people might say that you are going to regret not knowing Ruby. Rails implements a lot of Ruby's metaprogramming features, so knowing what is actually going on in your Rails app requires some fairly advanced Ruby knowledge.
Another point is that the Rails framework implements the Model/View/Controller design pattern. Models are objects. Controllers are also objects. You simply can't implement Rails without writing objects and calling methods on those objects. Most of what you do in a Rails app is write methods. If you don't know what an object is, or what a method is, you might struggle with this. It might be good to understand objects before you try to understand a particular pattern of objects. On the other hand, it might be good to get a strict introduction to how to use a very specific pattern of objects before you try the general case.
On yet another hand, in my own experience as a guy who doesn't know Rails, not knowing Rails is not much of a barrier to writing applications using Rails. It's probably a barrier to writing good, well-designed applications in Rails, but if you know Ruby well, you can hack something together using any Ruby framework.
All of that said, the advice I would trust most on this is the advice at the Rails Tutorial Book website. The best piece of that advice is this:
a surprising number of beginners have used this tutorial to learn web development, so I suggest giving it a try
If you get lost, go back and learn some Ruby, or some HTML, or whatever you're lost on, and try again! Learning Rails first isn't going to stop you from learning Ruby later, and if you find that you need more Ruby, you can always put Rails down and pick it back up. There's no wrong way to do this–do whatever keeps your enthusiasm for learning up, and whatever keeps you moving forward. Expect this to be pretty hard, though, whatever route you take. If it was easy, they wouldn't pay us to do it.