In short, the meme is a video of a group of people performing a skit to the 2012 song "Harlem Shake." The video is very short, about 30 seconds, and uses only a snippet of the song. Generally, one person, or a small group (often with someone wearing a helmet), dances to the song. Then, when the bass "drops" the video cuts to many people dancing. The Baauer song samples (and is named for) a song which mentions the 1980s dance moves popularized by Al B.
Here's an explanation from Chicagoist:
In a nutshell, a typical Harlem Shake video features about 30 seconds of Baauer’s song “Harlem Shake,” with one person in a group, usually wearing a mask, dancing awkwardly. When the break drops and the song says “Do the Harlem Shake” at about the 15 second mark, everybody flips out and starts doing the dance, which was actually popularized in 1981. This appearance of this zeitgeist can be traced all the way back to the last day of January, when a YouTuber called Filthy Frank posted a video of four dudes in spandex doing the Harlem Shake. Within days people were posting their own take on his video, and this century’s latest dance craze was born.
The original video does not have a crowd increase.
Here are some examples:
The people of Harlem, by the way, are not impressed. NPR's Ann Powers has strong words too:
"The deracination of the Harlem Shake has offended some who value the dance's origins in Harlem itself, where it's been done on the street since 1981, and its prevalence in hip-hop videos by the likes of Diddy and Jadakiss."Something important to remember before you blindly sign up for your office's rendition.
Harlem Shake (meme), Wikipedia
Why We Love Harlem Shake, Chicagoist
Harlem Reacts to 'Harlem Shake', YouTube (Uploaded by Schlepp Films)
Long Before The Harlem Shake, We Did The Shimmy, NPR
Behind 'Harlem Shake' Craze, a Dance That's Decades Old, The New York Times
Harlem Shake: One Name, Two Separate Dances, The New York Times (Video feature)