A teleconverter is a small optical unit that fits between the camera body and the lens. It makes the focal length of the lens longer, so distant subjects appear larger in the frame. It does not change how close or how far the lens can focus.
A 1.4x teleconverter on a 200mm lens gives a focal length of 280mm, but you lose one stop of light. A 2x teleconverter turns the same lens into a 400mm lens and costs two stops. Teleconverters can also slightly reduce sharpness and slow down autofocus.
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