This is a low cost LDR (Light Dependent Resistor). Basically, it's a light sensitive resistor; the resistance will decrease when the surface is exposed to light. It is widely used to detect day or night; for example, you could use it in an automatic porch light. It is a nonlinear sensor; when it is not exposed to light, the resistance goes up to 500K ohm, but from our testing, in normal room condition, it will be 10 to 20K ohm when the the lights are out. When illuminated, the resistance drops to 1K or less.