What exactly are tele converter lenses?

ive seen these on websites and they look interesting considering the price that they are, do i attach them to my lens or how do they go on? also if i was to get a x2 optical zoom one is that a long focal length, i wanna attach this to my 18-55mm or 70-300mm, also by the way i have a nikon d5000, and does it still let my camera autofocus?