What does a1 stand for?

According to the Kraft A-1 Steak Sauce web site:

In 1824, a chef to England's King George IV named Hendersen William Brand created a special sauce for the king. The monarch was so impressed with the new sauce that he proclaimed it "A1".

You can find the rest of the story at http://www.a1sauce.com/product_main.htm#history

Alternative Answer A is the first letter of the alphabet

1 is the first whole number So when you say somthing is A1 you are saying it is the first, the winner, the champion.