Teleconverters allow increasing the apparent focal length of a lens, giving a greater telephoto effect than a lens alone. They are basically a magnifying lens that is placed between the camera body and lens. They change the focusing distance of the lens only minimally by adding their length onto the focusing distance (from the plane of the camera sensor to the subject is the focusing distance).
Teleconverters have a few drawbacks. First, they dull the image making it appear not as sharp, though sometimes this is negligible. Second, they consume half the light as a best case so shutter speed will need to lengthen or ISO will need to be increased. Third, autofocus performance is compromised but with modern cameras this can be negligible, too. Fourth, they may introduce some aberrations. Fifth, the greater the magnification, then the greater reduction in contrast there is.