I have been confused a lot by the word "Ground". The ground that I've usually heard about is connecting a wire of appliances with the rod buried under earth but what does the word Ground mean when it comes to small amplifier circuits. If possible could you explain it with the circuit given below?
Ground means several things in a circuit like this.
First, the line marked with the ground symbol is usually connected also to the metal case that houses the circuit. This shields the circuit from picking up stray noise, radiated interference, and so forth.
You will also note that there are two lines marked with the ground symbol here. This means that those branches of the circuit are completed (i.e., current can flow through them) by means of their mutual conection to the common ground.
Finally, in a bipolar output stage like this one, the ground line serves as the zero voltage reference for both the B+ and the B- supplies and the current return for the load. So then the NPN transistor turns on, it is pushing current through the loudspeaker and into the ground, and when the PNP transistor turns on, it is pulling current up through the loudspeaker from the ground.