LiDAR (an acronym for light detection and ranging) is a technology that measures the distance to a target by shining a pulsed laser light on it and analyzing the reflections.
The surface that you are bouncing the laser light off of will impact how good of a reading you get. Smooth, reflective surfaces will tend to bounce the light away from the sensor depending on the angle at which the light is hitting the surface. If the light is coming straight at the surface, it will reflect directly back and you will get a good reading. The further from straight-on you get, the poorer of a reading you will get, eventually you'll get no reading at all. Glass, mirrors, and any highly polished surface will be have this way.
If the surface is rougher (i.e. not smooth) light will reflect off in various directions, some of it making it back to the sensor and giving a reading. Examples are paper, wood (not highly polished), and pets.
For more information, see the Wikipedia page on reflection.
LiDAR is often seen used with a rotating sensor that takes distance readings at various angles as the sensor assembly rotates. This provides distances in a circle around the sensor. This data can then be used to construct 2D maps.
Autonomous vehicles (i.e. self driving cars) often use LiDAR to map out the space around them.
The Garmin LiDAR Lite is a simple, non-rotating sensor that measures the distance to the target it is pointed at.
In this guide we will build a range finder, or electronic tape measure if you like, using the new Garmin Lidar-Lite. This is a laser based distance sensor that works by bouncing a modulated laser beam against an surface and measuring how long it take for a reflection to come back.