A photoelectric cell is a device that releases electricity when it is struck by light. An electric eye, which automatically opens a door as you approach it, is a good example of a photoelectric cell at work.
As you approach a door with an electric eye, you cut off a beam of light trained on the photoelectric cell. Then, as you continue walking and leave the beam of light, it again strikes the photoelectric cell. When it does, the cell sends out an electric current that swings open the door.
This same principle can be used to set off a burglar alarm and to turn on a hand drier in a public restroom. In fact, almost anything that runs on electricity can be operated by means of an electric eye!