For some context on why this is interesting: since the dawn of radio, we've known that relatively low radio frequencies (up to maybe 30MHz, depending on time of day, solar activity, and other factors) can propagate for very long distances, by following the earth's surface or bouncing off the ionosphere. This is why a portable shortwave radio can pick up radio stations from the other side of the world.

Higher frequencies don't benefit from these effects, and generally don't propagate beyond direct line-of-sight. But in rare circumstances, atmospheric refraction can cause high-frequency radio signals to propagate much farther than normal, which is what seems to have happened here.

On a technical level, this radio contact was made possible by the recently-developed FT8 digital modulation scheme, which is designed to transmit a minimal amount of data (basically just a pair of callsigns) at only a few bits per second, so as to make even extremely weak signals detectable. The "dB" column in the screenshot illustrates that the received signal strength was roughly 100 times weaker than the background noise.