Power Pills are the large blobs in the corner of the screens of a game of Pac-man. When Pac-man (or any member of his family) eats one of these, he goes OTT and starts chasing after the flashing ghosts. These ghosts are Pac-man's enemies, and usually chase him: but when he eats a power pill he becomes supercharged and can chase and eat them.
Of course, these pills have been associated with the drugs culture of the 90s, and the idea of Pacman taking pills on a regular basis was probably a Bad Thing.
They've also been insinuated in how Pac-Man and Mrs Pac-Man have sex