The 2x teleconverter (also known as a tele-extender) will multiply the focal length of the existing lens by 2x, and will cut back the amount of light by 2 f/stops. For example, using a 2x teleconverter with a 300 mm lens will effectively increase the focal length to 600 mm. A lens set to f/2.8 will now be f/5.6.
Teleconverters are not recommended for lenses wider than 50mm, e.g. 28-105mm and full AF operation is only possible with lenses with open aperture of f/2.8 or brighter. Even on lenses at f/2.8, autofocus cannot be guaranteed; manual focusing is recommended when using this teleconverter.