What Is A Teleconverter?
A teleconverter is an optical accessory lens that is mounted between the camera body and a regular lens. It has the effect of increasing the focal length of the lens, but also simultaneously decreases the maximum aperture of the lens. The most common focal length multipliers for teleconverters (sometimes called extenders), are 1.4x and 2x, although Nikon also makes a 1.7x version as well. Not all lenses are physically capable of accepting a teleconverter, due to the protruding optical element that actually reaches inside the regular lens. Most long lenses can accept them, most wide angle lenses cannot. Check with your manufacturer before purchasing! Linked below is an extensive article on teleconverters which includes best practices, when to use them, when not to, and also reference tables of extended focal lengths. When used correctly, and with their limitations in mind, these are incredibly powerful tools!
A 300mm lens with a 1.4x teleconverter becomes a 420mm lens. A 70-200 mm zoom becomes a 140-400 zoom when used with a 2x teleconverter.