It's people on the subway with no pants on. They have underpants on though, so don't get any ideas.
This past weekend was Improv Everywhere's No Pants Subway Ride, when New Yorkers brave the cold of winter and the stares of fellow transit-riders to use the subways in their underpants. The video above, from last year's event, is great primer (with butts!) of how it works. You can also see roundups with photos here and here and read about the event (with tons of photos) on Improv Everywhere's website.