LDR - Light Dependent Resistor
An LDR is a resistor that has a resistance value that changes according to the amount of light energy that falls on it. The symbol has arrows pointing towards the resistor rectangle, representing light photons falling on the LRD.
An increase in incident light energy releases more electrons within the component and this increases the current for any given voltage across the LDR. This means that as the illumination increases the resistance of the LDR decreases.
In the dark its resistance is very large (millions of ohms), in bright light its resistance is small (tens of ohms).
LDRs can be used as light sensors. They can be used to control the output of a circuit depending on the lighting conditions - to automatically switch on a lamp at dusk, or set off an alarm if an object is illuminated.
Very little current will flow through the LDR in the dark. It therefore can be made to work like a switch in a circuit. The 'switch' is closed in bright light and open in the dark.
Consider the potential divider circuits below. Make sure you can interpret such diagrams and understand how the physical conditions (how dark it is) affect the sensor's resistance and hence the share of the voltage it gets.