The previous reply makes no sense to me.
Actually the Arduino Vin pin can act as a external voltage source input to the Arduino board, or as a voltage output source to power external components.
Using the Vin pin as a output source the voltage will be .7vdc less then what is plugged into the Arduino external power connector, and be current limited by either the current rating of the external power source or the 1 amp series polarity protection diode on the Arduino board, whichever is less.
Either way, by way of the Arduiono external power plug or by wiring a external voltage source (of greater then around 7.5vdc) to the Vin pin, either will power the input to the Arduino on-board +5vdc voltage regulator to power the boards +5vdc components.
Lefty