its a parameter of an HTML tag that denotes the order when tabbing through links, fields on an HTML page. Its why when you're entering your information on a credit card info page, you can enter your name, then press the tab button to get to the "address" field, then to the "city" field, because "address" has a tab index that is the next highest from the "name" field, and "city" the next highest from the "address" field.