Teleconverters - also known as telecouplers and tele-extenders - are “in-between” optical gizmos that multiply the effective focal length of the lens attached to their front.  They offer the promise of a much-more-telephoto lens for far less expense than an additional lens - especially a long telephoto.  Now, they generally come in 1.4x and 2x variants.
   My introduction to teleconverlers was back in the film era - when my mother was offered a 2x one by a neighbor who shot weddings as a sideline; the price - back when the only electronics that my mother’s Konica SLR and the lens had to communicate with each other was aperture - was simple and the price tempting.  So she bought that doubler - and, used with the f/1.4 normal-length lens that she’d gotten with the camera - results were great. 
   Teleconverters now must do far more.  In the dSLR era, that starts with the camera’s autofocus system - which is one weakness of today’s teleconverters.  Another is that a teleconverter reduces light passing on to the camera; while that may never have been noticeable with that f/1.4 normal-length lens on my mother’s film SLR, those who’ve tried doublers now find the loss of two stops very noticeable - and manufacturers warn that autofocus systems may only function with certain-aperture lenses or wider.  
   A 1.4x teleconverter reduces lens aperture by only one stop - but a 2x teleconverter reduces it by two stops.  Depending on what lens you attach to its front - and what dSLR you use - this plays havoc with your dSLR’s autofocus system.
   Teleconverters only work with specified lenses - on a list the manufacturer includes.  Others may physically interfere - and damage - optical elements of the lens, the teleconverter, or both.
   Finally, need it be added that - if at all possible - a longer “real” lens produces better results?  If you can zoom out your zoom further, don’t shoot with the teleconverter unless it’s already attached.
   Teleconverters are perhaps most suited to photojournalism - where their small, lightweight aspects that makes possible much longer focal length can be a godsend.  Some dailies actually put one in every photojournalist’s bag.  One famous use by a press photographer was in getting a photo of the rifle abandoned outside a Colorado movie theater after a mass killing inside; the teleconverter allowed shooting the photo from an upper floor of a nearby building.
   Of course, you get what you pay for in teleconverters - as in everything else; Canon’s are weather-sealed, but the “everybody else” aftermarket ones don’t appear to be.  And Canon’s are substantially-built - while the “everybody else” aftermarket ones don’t appear to be what you want your  $3,000 lens anchored by.
   I’ve experimented with my Canon 1.4x - which I bought for an impending story - and results are great.

1.4x Canon telecoupler - now about $220-250 used.