its the guinness widget.
apparently quite the amount of money was spent on r and d to get the can pour the same as a pour from a tap. when the can is pressurized, it forces some of the black stuff inside, and then when the can is opened, the pressure in the can reaches atmospheric pressure quickly. The beer that was forced into the widget gets forced out through tiny holes and produces many many little bubbles similar to what a tap would do.
it short, it tries to help you pour the perfect pint of the black stuff.