A tele-converter is a "lens" which goes on in between the camera and the main lens (see image below), and it will double (or triple) the original magnification of any of your lens, similar to what a barlow lens does to eyepieces with telescopes.
They come in 1.5x, 2x, and 3x magnification.
There is one disadvantage with them though: they not only increase your magnification by 1.5x, 2x, or 3x, but will also increase your F/stop by the same amount!
(i.e. With a 1.5x converter on, F/3.8 is actually F/5.7, with a 2x converter, F/3.8 = F/7.6, and with a 3x converter, F/3.8 = F/11.4 etc) You will also lose a little bit of quality in the photos as well, but not much.
You can pick these up reasonably cheap, around (NZ) $30 - $60. Check with your local photo shop, they might have one in stock, if not, they should be able to order one in for you.