For people too lazy to click links, Guinness calls it a rocket widget. It is full of nitrogen and due to the balance of pressure between the nitrogen in the widget and the beer in the can, it stays sealed until the can is depressurized (opened). Draught stouts on tap are pressured by nitrogen gas, so the widget simulates the taste (texture?) of an on-tap draught stout with nitrogen bubbles.
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http://en.wikipedia.org/wiki/Widget_(beer)
Widgets!
Short answer: it makes the beer taste more like it came from the tap.