a feature of some cameras, notably professional SLRs.
When a SLR takes a picture, the mirror flips up, out of the optical path. Once the mirror is flush against the focusing screen, the shutter starts opening.
After the shutter has completed its cycle (and the film has been exposed), the mirror comes down again.
The mirror has to move up quite rapidly, in order to minimize the delay between the moment in which the button is pressed, and the moment when the film is exposed. This delay, analogous to lock time, can be very annoying.
On the other hand, a mirror that moves very rapidly gives a healthy whack against the camera frame, which translates in camera vibration.
In most situations, this can be ignored - the effect is lost in the noise of hand tremor.
But when you are shooting from a tripod, especially macro, it turns out that mirror slap is the single most important source of vibration. Remember also that a tripod usually does not dampen very well camera vibration.
Medium format reflex cameras (like most Hasselblads) have particularly large mirrors (big lens, big film, big camera imply big mirror), which means that the slap is even more of an issue.
That is why for macrophotography it is very handy to be able to lock the mirror in the up position: mirror lockup.
Some cameras lack true MLU, but they do raise the mirror at the beginning of the self-timer delay. This is a useful trick to simulate MLU - unfortunately excessively smart modern cameras have lost that trick.
Some wideangle designs require the last element of the lens to be very close to the film, actually closer that the mirror allows. This is why some (usually old) wideangle lenses require mirror lockup.
minithanks to cbustapeck.