Microwaves aren't very smart. It generally has no idea what you're trying to heat up, and if it's (say) a burrito and not (say) a frozen turkey, a bunch of extra microwave waves are going to waste. This blue box can suck up all of those spare microwaves and turn them back into electricity that — wait for it — can power your microwave again.
Dengyo's "Microwave Regenerative Converter" can be installed in your microwave, where it will harvest a significant amount of the microwave energy that doesn't get absorbed by whatever you're trying to defreezerfy and turn it straight back into electricity. Up to 100 watts can be recovered in this way, which might be around 10% of the energy that your average microwave emits.
This is certainly not the first attempt at harvesting energy from wireless signals, but the advantage of using a microwave is that you're dealing with very high energy waves in a very small space, which means that you can expect to get a reliably substantial amount of power back. The more powerful your microwave is, the more electricity you'll recover, and Dengyo plans to try this device out first in "industrial microwaves," which is probably not what you have in your kitchen even if you use it for things that you shouldn't.