A micrometeoroid was a small meteoroid-like fragment floating through space. Micrometeoroids posed a hazard to spacecraft, causing damage to the hull of a ship due to the impact of the rock particles against the hull at very high velocity. To avoid such damage, a ship must be equipped with some type of deflector shielding to prevent the rock particles from hitting the hull.
The USS Voyager encountered a micrometeoroid shower in an alternate 2374 timeline in the Delta Quadrant. Voyager's outer hull, already severely damaged from incessant attacks by Krenim warships, was damaged even further by the shower. Captain Janeway managed to manually get the ship's shields online to protect it, at the cost of severe injuries to herself. (VOY: "Year of Hell, Part II")