Rupa is form or matter; something material that can be sensed. In early Buddhist literature, rupa includes the Four Great Elements (solidity, fluidity, heat, and motion) and their derivatives. These derivatives are the first five faculties listed above (eye, ear, nose, tongue, body) and the first five corresponding objects (visible form, sound, odor, taste, tangible things).
Another way to understand rupa is to think of it as something that resists the probing of the senses. For example, an object has form if it blocks your vision -- you can't see what's on the other side of it -- or if it blocks your hand from occupying its space.