They recreated the most iconic music videos of the year (Sorry, WFH, Famous, Hold Up, STS, TIWYCF and others, MVs in the end are most viewed videos on YT) with famous youtubers (or at least I guess so) but also including other relevant/viral things about 2016, it's not that hard to get.
Wow, my kings, the Dolan Twins, being in the end with the carpool karaoke bit. After being on an international tour and topping itunes with their album. This really was their year