The Arduino Uno contains a built-in Analog-To-Digital (ADC) converter with six channels, which gives us six analog input pins. The ADC converts an analog signal into a digital value. While the digital pins have two values, either high or low, the analog input pins have values from 0 to 1023 relative to the reference value of the Arduino. The Arduino Uno has a reference value of 5V.
The analog input pins are used to read analog sensors such as rangefinders and temperature sensors. The six analog pins can also be configured as digital pins if we run out of digital pins in our project.