What do we mean by ground in an electrical circuit? When we want to solve electrical circuits problems, we set a "ground" level which is mean that the potential on it equal to $0$. I wonder how the current will flow if the potential at a point equal to zero.
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.
I agree with Passerby, but Passerby's answer describes the term neutral. There is one more use for the term ground, which differs from the term neutral - The ground should not change it's potential when a a (large) current is passed to it, the potential of the ground must be stable.