The Canon Extender EF 1.4x III is the last version of the extenders that Canon has brought to the market. A 2x version is also available.
Very simply, it means that your (zoom) range is increased by a factor of 1.4. In combination with an APS-C sensor with a crop of 1.6x, your range can become considerable that way. eg. a 400mm lens can then (theoretically) have a range of 400 x 1.6 (crop) x 1.4 (extender) = 896 mm.
These extenders are specifically designed for Canon's (professional) EF lenses, especially the (super) telephoto lenses, both prime and zoom lenses.
This device weighs about 200 grams and is (very) solidly built, comparable to the L-lenses, ie in metal with optically high-quality materials.
Attention: if you place this extender on your camera, it will affect your autofocus system. You fall back one stop in aperture, so for example from f 5.6 to f8. Only Canon's more recent cameras keep their autofocus system at f8. eg. the 6D, 77D, 800D, 80D,.... Not the older versions. In addition, your camera should not only be compatible with this extender, in addition, the number of focus points of the camera/lens is limited by the extender. With certain (older) lenses, for example, you only have one autofocus point, of which the autofocus system is then (strongly) slowed down.
The newer lenses are more compatible with the canon extenders. The Canon EF 100-400 mm f/4.5-5.6L IS II USM is a good example of this. Combined with a 6D or 77D, you still retain a large number of autofocus points in the middle, without losing too much speed in the autofocus system. Both are, as it were, (better) attuned to each other.
In short: this extender is therefore especially interesting for a compatible camera and preferably combined with a lens with a high aperture (f2.8, f4,...).