Unlike some other devices the Raspberry Pi does not have any analogue inputs. All 17 of its GPIO pins are digital. They can output high and low levels or read high and low levels. This is great for sensors that provide a digital input to the Pi but not so great if you want to use a sensor that doesn’t.
For sensors that act as a variable resistor such as LDRs (Light Dependent Resistors) or thermistors (temperature sensors) there is a simple solution. It allows you to measure a number of levels using a single GPIO pin. In the case of a light sensor this allows you to measure different light levels.