A stylus is a plastic tipped pen-like writing utensil which is used to write or manipulate objects on a capacitive touch screen. Before ereaders and tablets, a stylus was the number one accessory used to navigate a personal digital assistant (PDA), the original portable computers and ereaders, such as the Blackberry and similar devices. While very few tablets come with a stylus today, they are an optional accessory which can be purchased for nearly every ereader on the market.
Using a stylus isn't usually necessary, just a personal choice. For individuals who have difficulty using a touch screen or like writing notes in the margins of books, a stylus is likely the right option for you.