Somewhat lose term to define a paved road not close to urban agglomerations and devoid of urban traffic. This implies absence of roundabouts, traffic lights, heavy traffic and so on. Open roads usually have only one lane per direction
The term usually (but no obligatorily) can imply rough tarmac, sand on the road and potholes.
Commonly used by cyclists, riding on the "open road" also can imply being more exposed to the weather and being far away from help if something goes wrong.
"I expect to get at least some couple thousands of kilometres of training on all weather open road conditions out of this hub"