I think she is talking about a tele-converter, aka extender.

You can get 1.4x and 2x.

With a 1.4x a 200mm lens becomes a 280mm

With a 2x it becomes a 400mm.

However, there is no such thing as a free luch and with a tele converter, ther is a light loss to deal with;

a 1.4x loses 1 stop, so a 200mm f2.8 lens is now a 280mm f4

a 2x, 2 stops, so the 200mm f2.8 is a 400mm f5.6

Now if your using a consumer zoom like a 75-300, which has a max aperture of f5.6 at the long end, this would become at the long end either 420mm f8 or 600mm f11.

most SLRs require a max aperture of f5.6 for the autofocus to work, so you will probably lose the AF and f11 will require long exposures and a decent tripod to avoid camera shake.

There is also a loss of image quality, more with a 2x than a 1.4x

with a digital SLR you are often better just cropping than using a teleconverter