Apollo 11, 14, and 15 each deployed arrays of corner-cube prisms on the lunar surface. These aren't mirrors in the traditional sense—they're specially designed optical devices that reflect incoming light directly back toward its source, regardless of the angle of incidence. The Apollo 11 array contained 100 fused silica cubes, while Apollo 14 and 15 deployed larger arrays with 300 and 300 cubes respectively.
Observatories on Earth fire short pulses of laser light at the known coordinates of these arrays. The light travels approximately 384,400 kilometers to the Moon, reflects off the retroreflectors, and returns. By measuring the round-trip time (about 2.5 seconds), scientists calculate the Earth-Moon distance with millimeter-level precision. This technique is called Lunar Laser Ranging (LLR).