Of course, learning ROS without the robotics knowledge is useless. So how can a person independently learn robotics? While I will cover or point to some of the ideas of robotics, here are the resources you should use.
As I recommended before, I think Artificial Intelligence for Robotics class in Udacity is really helpful in developing the understanding of the essence of robotics (if I had any sense of it at all). I found that standard robotics class or courseware focuses too much on the mechanics of the robot rather than its intelligence (joints and angles everywhere!). Even if you are not familiar with the programming part of the Udacity course, skimming through the idea part of the course is, I think, a good idea.
ROS wiki itself is a good resource. Go through each of the packages in our code repository and read what each package do – it is easier than you would think.
ROS have truly transformed the job of the programmers in the robotics field, as much of the robot functions are implemented in the aggregate of the ROS libraries. Just as programmers don’t necessarily need to understand the string representation in a language to use it, robotic programmers do not need to know the implementation details of each and every robot parts – just of its utility.