Mirror lock-up raises the SLR’s mirror before the exposure so the vibration from the mirror flipping up doesn’t blur the image. On many cameras this happens with a self-timer or with a first press to raise the mirror and a second press to take the shot.

Its main use is reducing camera shake during tripod-mounted shots at slower shutter speeds, especially around the range where mirror slap vibration is most noticeable. It matters far less at very fast shutter speeds, and often also less for very long exposures once the vibration has settled before the shutter opens.

Use it when you want maximum sharpness in situations like landscapes, macro, telephoto work, or other critical exposures on a stable support. Pairing it with a remote release or self-timer helps even more.

Strictly speaking, some cameras have true mirror lock-up, where the mirror stays up until you release it, while others have a mirror pre-fire mode just for the next exposure. The purpose is the same: separate mirror movement from the actual exposure to minimize blur.