LIDAR is an acronym for
LIght Detection And Ranging, coined by analogy to RADAR, for Radio Detection and Ranging, for Lidar is pretty much Radar done with a laser beam rather than radio waves. A low-power laser beam, usually in the infrared or ultraviolet wavelengths, is scanned rapidly back and forth sort of like the electron beam in an old-fashioned television screen. With each pass over the scanned area, any reflections back are detected and measured to give a readout of range to target. This builds up a very accurate and complete 3-dimensional picture of what's out there in the scanned volume of space. Range is limited by the low power of the laser and speed-of-light lag to a few light-seconds, but within that range the location of objects and their speed can be determined with a high degree of precision. As with any active sensor, Lidar can be detected by passive sensors a lot further than Lidar can detect something using only passive sensors. For these reasons, Lidar is mostly used not for the initial detection of targets nor for general scans of an area, but for specialized tracking of targets initially detected by other means, and the direction of weapons onto that target. Most point-defense weapons use Lidar for targeting purposes, most missiles and lance torpedoes use Lidar for terminal guidance, and and starships use Lidar for close maneuvers such as docking with a space station.
Lidar is, of course, a real-world technology.