What do we mean by "ground" in an electrical circuit?
Ground is an arbitrary definition for the (typically) lower side of a voltage differential. It's normally considered 0V, but since a voltage source is simply a difference in voltage, it's in relation to the other voltage potential. For example, two 9V batteries in series could be 18V to 0V, or can be 9V to 0V to -9V. It's all relative to what you decide is the reference voltage, aka ground.

Current flows from one voltage potential to another. If there is no difference of voltage between two points, current will not flow.