SLR stands for single lens reflex; and the reflex part refers to the use of a mirror to reflect the image to a viewfinder. There are also TLR (twin lens reflex), and DSLR/DTLR with the D added for digital.
That is opposed to non-reflex designs like large format where the "viewfinder" ground glass has to be removed and replaced with the film tray. Other simpler designs like rangefinders, and newer mirrorless cameras also do not use a mirror and are not relex designs.
The Canon lens mounts for film cameras is/was screw, R, FL, and then FD. EF followed with electronics for communication with EOS cameras (film/digital).
Currently there is EF (-S small image circle), *EF-M (M series mirrorless), and RF (R series mirrorless, -S small image circle).
The only time Canon didn't require you to buy all new lenses as you upgraded/changed was when they first replaced the film with a digital sensor (original EF mount). If you already have full frame lenses you do not have to replace them if you "downgrade" to a small sensor camera (of that series).
(*only third party are still making EF-M lenses)