The universal product code (UPC) is the official name for the pattern of black lines on the side of almost everything you can possibly purchase. Simply put, the bar code.

What makes the bar code interesting is that the lines as well as the 12-digit code have meaning. The white spaces between the black lines are also meaningful. The bars are for machines to “read,” the numbers for us unsophisticated humans. We’ll get to the bars in a bit.

The first six numbers are related to the manufacturer and the type of transaction, the next five describe the product or item number. The last number is the checkdigit, which is used to validate that all of the other numbers have been read correctly by a scanner.
The first number, set off from the rest on the left, designates the general nature of the purchase; coupons, pharmaceuticals, or special pricing arrangements made within the store among other possibilities. The next five numbers, grouped together, are the specific manufacturer ID. The next three are called the family code, and the two digits before the checkdigit are the value code. The family code describes a family of products, while the value code designates the value or nature of a coupon.
Now, the bars. Notice that there are four different widths of bar in any UPC code. Each of the four sizes corresponds to a value of one to four. So a black line followed by a white line followed by another black line is three “ones.” This is how all UPC codes begin. After this “start code,” every sequence of 4 line widths corresponds to binary code for one of the numbers that appear beneath the bars. The last three digits are once again three “ones.”
In mathematics and digital electronics, a binary number is a number expressed in the binary numeral system or base-2 numeral system which represents numeric values using two different symbols: typically 0 (zero) and 1 (one).

info from Dictionary.com