Some basic explanation:
The analog-to-digital converter (ADC) inside the microcontroller converts a signal from 0...5V to a value of 0...1023. To use the ADC, the function analogRead() is used.
The ADC is however capable of more than just that. It uses a voltage reference for the conversion. Normally that voltage reference is +5V. But it can be set to +1.1V.
With a voltage reference of 1.1V, the ADC converts a signal an analog input pin from 0...1.1V to a value of 0...1023. That's roughly 1mV resolution. So the accuracy increases. Nice!
This is all happening inside the chip.
The voltage reference is connected to the outside world with a pin. On your Arduino Board, you see "AREF", that's the reference.
If you switch to another voltage reference, it is best to wait for 20ms, to let the voltages settle.
void setup()
{
analogReference (INTERNAL); // 1.1V reference
delay(20); // let voltages settle
}
void loop()
{
int rawADC;
rawADC = analogRead(some_pin); // 0...1.1V -> 0...1023
}