A simple
photography tool that acts as a
barrier between
film and the
lighted world.
Dark slides are always present in any camera that has a detachable film holder such as many medium format single lens reflexes and all view cameras.
On your more sophisticated cameras that act as a system, this aspect has been idiotproofed so it is impossible to either remove the film holder without the dark slide in place, or depress the shutter with the dark slide still in. View cameras, being little more than a lightproof box with a lens at one end and a film holder at the other, and being generally assumed to only be in the hands of people who know what they're doing, never have such a feature.
This leads to numerous mistakes early on when learning how to use a view camera - quite frequently the photographer will forget about the dark slide entirely.
Opening the shutter with the dark slide in place will cause light to strike the barrier as opposed to the film surface giving you... nothing... for all your efforts. This mistake isn't too painful, as the film is not ruined and it's no big deal to simply take the photo again - if you noticed your error before reaching the darkroom, that is.
The reverse error of removing the film holder without replacing the dark slide results in the film being flooded by outside light - ruining the film entirely. Again, aside from the wasted product, this isn't too huge of a problem as only one sheet of film has been ruined, and it is nearly impossible not to notice right away.