An acid such as vinegar or lemon juice helps the whites to set faster so you don't have lots of wispy whites floating around, which are more common with older eggs. Simply add a splash of white vinegar or lemon juice to the water and stir before adding the eggs. However, you can poach eggs in plain water, which is what I most often do if I'm using very fresh eggs. For more information on how to poach eggs, check out this detailed post by Lisa.