I stole the image below from the first Google image I found. This isn't a recommendation for this particular brand, it's just for illustration.
Basically, disc brake pads have a metal backing plate, and the actual pad material is bonded to the plate. Manufacturers will specify a minimum thickness of the backing plate + remaining pad material, which you would measure by calipers. In contrast, with rim brake pads they can just mold a wear line on the pads, but on disc brake pads such a line wouldn't be easily visible.
The pads in the original photo don't look like there is any pad material left at all, rendering the question of minimum thickness moot except for future reference. Here, the pad holders themselves could have been scraping against the disc rotors, which is bad for the rotors (and for reference, the rotors also have a minimum thickness). TL;DR: they were worn out some time ago.
You can ask a bike store to perform periodic inspections. Alternatively, many of us learned to read basic vernier calipers in high school, and a pair accurate to 0.1mm isn't very expensive. A small step up would be cheap digital calipers that produce automatic readings and can measure to 0.01mm.