It's really cool tech. The retroreflectors have been up there for what, 30+ years?, and they've been bouncing lasers off them for as long as they've been there. Prior to my wife's program, APOLLO, they were measuring the distance to the moon down to the centimeter level. With APOLLO, they're down to the millimeter.

There are five retroreflectors on the moon, three left by Apollo, two by Russian robotic probes. One Russian reflector is unreachable, it either is not in the location that it was thought to be at, or perhaps the support brackets for the reflector broke and it's no longer properly aligned. The other Russian reflector can only be used when it's in the dark: when the sun is shining on it, apparently the mirror flexes and is out of alignment.

The laser beam is 3.5 meters exiting the telescope, 2 km when it hits the moon, no idea how big it is when it gets back to earth. And they have to hit these little 1ish meter targets pretty much dead-on to get a return. They mentioned that the laser sends out a quadrillion photons per pulse, only getting a handful in return. What they didn't mention in the episode is that the laser is pulsed, sending out multiple pulses per second (I don't know the exact pulse rate), it has to pulse as the same apparatus has to listen for the return when it's not transmitting.