What it does is easier said that explaining how it does it!
Used with EF USM lenses in AF mode, DEP/ADEP sets a hyperfocal distance at a given combination of aperture and shutter speed. Simply, it means when you have decided on your subject, typically a landscape subject DEP mode, you indicate to the camera the first point of the scene you wish to have in focus (e.g. Ma Blog in the foreground); then indicate to the camera the second point (the Tree of Content behind Ma Blog, maybe). Recompose and the camera has now set a hyperfocal point to get everything between those points in sharp focus/or depth of field (technically, the range of acceptable sharpness). Experimenting and taking notes of what you are doing will be very helpful. Beginners have trouble trying to understand why little happens with ultra-wide angle lenses e.g. 20mm. Those lenses have inherently great depth and won't really benefit.
I used DEP mode on my EOS 5 frequently in 1995-1997 when coming to grips with hyperfocal points was a bit of a challenge to understand! I now routinely use implied hyperfocal on my TS-E 24mm lens (manual focus) but not at all now with other EF USM Canon lenses.