ISBN stands for International Standard Book Number.
Up until a couple of years ago, all ISBNs were 10-digit.
Let's take the ISBN for How Do Dinosaurs Say Goodnight:
The first digit tells you what language the book was published in. A zero or a one means English. (The ISBN for Bonne Nuit Petit Dinosaure is 2-0705-5568-2. The first digit, a two, means it's in French.)
The next four digits tell you the publisher. 5903 is one of the codes for Scholastic.
The next four digits are a random series of numbers, unique to the book.
The last digit is a check digit. In any computer set up for ISBNs, the computer can run through a mathematical calculation using the check digit to be sure there isn't an error in the previous series of numbers.
(If you're curious, you multiply the first digit by ten, the second by nine, the third by eight, the fourth by seven, the fifth by six, the sixth by five, the seventh by four, the eighth by three, the ninth by two. Then add all those products together. In the case of 0-5903-1681-8, you'd end up with 190. Then you ask yourself how much you would have to add to 190 to make it a multiple of eleven. The answer, and thus the check digit, is eight.)
(If the answer were ten, then the check digit would be the letter X.)
A couple of years ago, it started to look like we might run out of unique ISBNs, so the whole world switched to 13-digit ISBNs, which was accomplished by keeping all the 10-digit ISBNs, but adding 978 at the beginning. (Of course this necessitated a different check digit.)
The barcode shown here has two parts. The big barcode will tell a scanner the ISBN, which you'll see printed in its 10-digit form and 13-digit form above the barcode, and in its 13-digit form again under the barcode.
The little barcode will tell a scanner the price. You'll see the price printed twice above the little barcode: once with a $ in front of it, and once with a 5 in front of it. (5 is code for US dollars.)