A voltage reference is an electronic device that ideally produces a fixed (constant) voltage irrespective of the loading on the device, power supply variations, temperature changes, and the passage of time. Voltage references are used in power supplies, analog-to-digital converters, digital-to-analog converters and other measurement and control systems.
Voltage reference - Wikipedia
You could check how it is done technically, but you don't have to worry about that. It is something internal to the chip and it's used by its circuitry, not by you.
You have to focus in the specs of the ADC, how to use it, in the datasheet.
Which module do you have? ESP32, ESP32-S3, S2, C3...?