The way your camera works is this:
The light enters your lens, hits a mirror, then it bounces up into a prism. The light then ricochets
off the prism (twice) and finally through the viewfinder and into your eye. It is this structure that
allows you to see the same thing your lens sees even though it is not aligned with the viewfinder.
This lens - to mirror - to prism – to viewfinder structure is what they are referring to when they
call a camera a SLR (single lens reflex) camera. A camera referred to as a DSLR is the same
thing only using a digital sensor instead of film.
The mirror (#2 in the previous illustration) is located between the lens (#1) the shutter (#3). To
allow the light to pass through the shutter, depressing the shutter button not only opens the
shutter, but also lifts the mirror up and out of the way (to position #5). After the shot, the mirror
falls back into place. (You can actually hear it flapping up and down if you listen for it.)
But, there’s a problem.
When the mirror is lifted there is a tiny bit of vibration and we all know what that means, it
means that lifting the mirror is inducing some – admittedly minor - camera shake.
Even when you are using a timed shutter release or a bulb release – the mirror still has to be
lifted out of the way.