It's a video game called "Dance Dance Revolution", where you have this huge pad and have to jump on arrows corresponding to the ones on the screen. Basically, it's an excuse to look really stupid while jumping up and down, which (generally) leads to everyone having a good time.

Some schools have installed DDR machines in their gyms because CHILDREN ARE INCAPABLE OF EXERCISING WITHOUT FLASHING SCREENS TELLING THEM WHAT TO DO.