Extenders, generally known as teleconverters, increase the effective focal length of lenses. Canon Extenders are available in two strengths, 1.4x and 2x. As the names suggest, the 1.4x Extender increases the focal length of your lens by a factor of 1.4, and the 2x by a factor of 2.
Canon EF Extenders are designed for use with a number of telephoto and zoom EF lenses. They do not work with EF-S or EF-M lenses. They can be used with a compatible EF lens on a Canon EOS R System camera with an EF-EOS R Mount Adapter. If you're using RF lenses, you need Canon RF Extenders, which are designed to work with certain RF lenses – more about these shortly.
Not all Canon lenses accept Canon Extenders. This is because of their construction – extenders have a protruding front element that will not fit into the rear of many EF lenses and some RF lenses. Compatible lenses have a recessed rear element, which creates space for the front element of the Extender. (In the case of the RF 100-500mm F4.5-7.1 L IS USM, you need to zoom to at least 300mm in order to move the rear optical elements enough to fit an RF Extender. It is not physically possible to use the Extender at the lower end of the lens's zoom range.)
Extenders are a relatively cheap and convenient way of enhancing your telephoto capability. But using them to increase focal length comes at a cost − reduced maximum aperture. The 1.4x Extender causes a decrease of one stop in the maximum aperture of the lens, while the 2x Extender causes a loss of two stops. This means you gain extra focal length at the expense of losing some light. If you attach a 1.4x Extender to an EF 70-200mm f/2.8L IS III USM lens, for example, it will become in effect a 98-280mm f/4 lens. If you use the same lens with a 2x Extender, it becomes a 140-400mm f/5.6 lens. In the same way, the EF 70-200mm f/4L IS II USM lens has a maximum aperture of f/4, but with a 1.4x Extender, the effective maximum aperture becomes f/5.6, while with a 2x Extender it becomes f/8.