When you set a tab stop in either the X or Y ruler, it creates a vertical or horizontal dotted green line in the Work Window at the pixel location of the tab stop. These lines are known as Tab Guides. (See Work Window.)
You can use Tab Guides to help make alignment of objects precise and easy. You can use them to align objects along their left, right, top, or bottom edges or their centers.
You can toggle the Tab Guides off or on by selecting View>Display Tab Guides, or by selecting/deselecting the Display Tab Guides checkbox on the Guide/Display Options tab of the Workspace Options dialog box. (For more information, see Guide/Display Options.)