In short words:

Yes you can use an arduino to work as a ROS node, there are good examples here wiki.ros.org/rosserial_arduino/Tutorials showing you, how you can use an arduino to subsribe and publish to messages f.e.

What you want to do if your motors are driven by an arduino, is to programm the arduino as a, i call it "base_controller". This way the arduino (running as ros-node) could subsribe to the topic "/cmd_vel" and then "translate the subsribed message into commands to steer your drives connected to it.

what i expect you to do first, is to go through the basic tutorials for ROS, so you understand the ROS infrastructure and what f.e. nodes are, what they do (subscribe and publish messages, in generall) and so on.

Hope this helps a bit, Fabian