Its a $26 small handheld 60-100x microscope with built in LED lighting.
The point of it is to examine the trichromes of your flowering plant to determine if they're mature, so you can tell whether or not your plant is ready to harvest.
Much has been written about this elsewhere, with pictures, but in a nutshell all clear trichromes mean harvest is premature. All yellow/amber trichromes mean you're past peak potency. What you want is some mixture of clear, cloudy, and maybe a few amber trichromes as a marker of maximum potency for harvest.
Note that THE EXACT SAME MICROSCOPE without the obnoxious "420" on it, is available as the "Carson MM200" for less money elsewhere ($12-20 depending on source).
There are other similar microscopes available as well, on Ebay, at Radio Shack, and elsewhere.
Personally, I find that a cheap 40X illuminated loupe (available on Ebay for $5 shipped) works great, and is a little more convenient to use than these sorts of scopes. But this is largely a matter of personal preference.