You'll see that they've created network diagrams for each House of Representatives from 1949 to 2011. They've drawn dots for each representative, and lines connecting pairs of representatives who vote together a given number of times. Finally, the dots for each representative are placed according to how frequently the Representatives vote together overall.
What we're left with is a picture of political mitosis. Similar voting between Democrats and Republicans was fairly common up through the 1980s. But starting in the 1990s the parties began pulling apart from each other, like a single cell dividing into two.
Not only that, but within parties Representatives are voting more similarly too -- that's illustrated with the dots in each party's cluster becoming more tightly packed together over time. Starting in the 2000s, there are hardly any links between the parties at all.
Again: this is nothing we don't know. In fact, historically our current era of polarization may just be a return to historic norms. And while this visualization is effective at showing the parties peel away from each other, it misses some other nuances about polarization -- for instance, that current trends are largely driven by Republicans moving away from the center.
Still, it's a great way of showing just how much Congress has changed over the past 60 years.