There is a very simple working principle behind this technology.
it is nothing but the technology of 1s and 0s.
in this a laser (mainly withe help of Bar code detection guns; a perfect transceiver) is made to fall upon the bar code sheet.
as we know the principles of BLACK and WHITE colors, the black color bar absorbs the laser, due to this at receiver side we can not get any signal. but the white color bars reflects the laser light, which are further absorbed by the receiver.
so, digitally we get 0 from Black color and 1 from white color .
so, further decoding this 0 and 1 combination, we can get the coded data into Bar code.