Teleconverters or Extenders are attachments placed between the camera and lens that increase the focal length of the lens by a given factor. A 1.4x teleconverter will give a focal length of 280mm if used with a 200mm lens; a 2x would make it a 400mm lens. The cost of high quality fast lens seems to increase exponentially with focal length (the Canon 800mm ƒ/5.6 IS USM lens costs $13-14,000) so teleconverters offer a much more affordable way to reach these focal lengths.
However, there is a catch: attaching a teleconverter will reduce the maximum aperture of your lens. A 1.4x teleconverter will reduce it by one stop (so an ƒ/2.8 becomes an ƒ/4); and a 2x will reduce it by two stops (an ƒ/2.8 becomes and ƒ/5.6). This means that you’re going to need a fast lens. The issue is not just longer exposure times and a loss of depth of field. The camera’s autofocus system will also require a sufficiently large enough aperture to work effectively.
If you’re starting with an ƒ/4 lens and using a 2x teleconverter, you’re now at ƒ/8 at which point most autofocus systems will not work and you’ll need to focus manually. Consequently, to use a 2x teleconverter you ideally need to have an ƒ/2.8 lens; for a 1.4x you could start with an ƒ/4 lens—at ƒ/5.6 autofocus will still work but the number and type of AF points will be reduced.