Take a CD or a glass prism . take it to near a source of light like a torch or a natural object that is sun . we will observe that when light falls on CD or the glass prism , it splits into many color. this process is known as dispersal .  you will also observe band of colors . this is called spectrum 
Take a glass prism and expose it to sunlight and adjust till you get the seven band of colours VIBGYOR. if there is no prism, then take a cd and expose it to sunlight or any other light soucrce and you'll get the seven colours dispersed. the splitting of the light to its constituent colours when passed through a prism is called dispersion of light.