Each letter of the alphabet is assigned a number. Alif = 1, ba = 2, etc.
Back in the old days, the Arabs didn't have numbers. They used letters. Later, they adopted the Hindu system, added a zero. The words "zero" and "cipher" are from Arabic, as are many of our mathematical words, like algebra.
Eventually, Europeans got tired of Roman numerals, and started using the system the Arabs had gotten from India. That's why we call the numbers we use "Arabic numerals."
Good thing we don't use Roman numerals any more. Care2 woouldn't allow us to count by tens: x, xx, %#&!*%, etc.