Clarification on the use of the pin AREF

Hello, my name is Raphael Campos.

I need someone to enlighten me the operation of the AREF pin. I read on the Arduino site this warning:“If you’re using an external reference on the AREF pin, you must set the analog reference to EXTERNAL before calling analogRead().”

I need someone to enlighten me this statement. Thanks.

Default setting is to use Vcc (normally 5V) connected internally.
If you want to use something else, such as an external precision 4.5V reference,

so you are not subject to Vcc variations from the USB source (5V +/-5% - or 4.75V to 5.25V) or to regulator variations, then you would select the External source.

Or if you wanted the 0-1023 range to cover 0-2.5V vs 0-5V if you had a lower level signal, such as as a line-level (+/-1V) signal that you had offset to 1.25V with a capacitor and voltage divider.