Ground is referred to a point in a circuit which has the following properties,
-It can sink or eat whatever current applied to it.
-Since it sinks the entire current applied to it, this point has no potential to generate voltage by sourcing current.
-Hence this point is said to have 0 V.
The ground is necessary in a circuit, since any circuit will work only if the current ‘flows’, and all the current flows from the positive voltage source to ground.
Again since the ground has 0 volt it can be used as a reference point to measure the voltages at different points in a circuit.
Ground is very useful while connecting two seperate power sources together.
Expecting more doubts from you!