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Light Dependent Resistors (LDRs) are little light sensors, and this is a BIG pack of 5 of them! As the squiggly face is exposed to more light, the resistance goes down. When it's light, the resistance is about 5-10K ohms, when dark it goes up to 200K ohms.
These are the same KLDRs we use in our popular Edukit #2 Sensors.
To use, connect one side of the photocell (either one, it's symmetric) to power (for example 5V) and the other side to your microcontroller's analogue input pin. Then connect a 10K pull-down resistor from that analogue pin to ground. The voltage on the pin will be 2.5V or higher when it's light out, and near ground when it's dark.