the values you set determine the order that your keyboard focus will move between elements on the website.
In the following example, the first time you press tab, your cursor will move to #foo, then #awesome, then #bar
<input id="foo" tabindex="1" />
<input id="bar" tabindex="3" />
<input id="awesome" tabindex="2" />
If you have not defined tab indexes anywhere, the keyboard focus will follow the HTML tags of you page in the order in which they are defined in the HTML document.
If you tab more times than you have specified tabindexes for, the focus will move as if there were no tabindexes, i.e. in the order of appearance of the HTML tags