Abbreviation: EF = "Electro-Focus" (Canon)

The abbreviations means "Electro-Focus"

Explanation from Canon

The original EOS camera system was introduced in 1987. At its heart was the EF lens mount, connecting lenses to the camera body in a fully electronic way for the first time. Apart from the physical connection between these two components all communication is electronic, allowing for highly accurate, real-time data transfer. Canon became the first to build the motor that drives the focus inside each lens, rather than using a motor inside the camera body. Even with continued development and evolution of focusing systems, Image Stabilizer technologies, specialist lens elements and weather sealing, the original EF mount design remains unchanged. All EF lenses are compatible with every EOS camera ever produced, including the new EOS M series when used with the EF to EOS M Mount adapter.

See examples of effected products

Canon EF 135mm f/2 L USM
Canon EF 300mm f/2,8 L IS USM
Canon EF 70-200mm f/2,8 L IS II USM

See links

https://www.canon.ie/lenses/tech-guide/compatibility/
https://en.wikipedia.org/wiki/Canon_EF_lens_mount
https://www.canon.se/lenses/tech-guide/compatibility/