Ground means the reference point in your circuit. All the voltages in the circuit are measured/refered with respect to that point.
Circuit Ground need not be at 0V with respect to actual "EARTH" ground which is considered to be at zero potential. So even if you say that some point in yor circuit is at ground level it may be at potential higher/lower than Earth level but for your circuit it is considered as 0V.
In short, ground is the reference point with respect to which all other voltages are measured or refered to.