ISTM that the OP doesn't need the divider, that he can use the 'secret voltmeter', and he has to do the calibration mentioned by mircho.
retrolefty is explaining that the reference for the ADC by default comes from the power supplied to the AVR chip, whatever that is. There is an option to use an external voltage on the AREF pin, which is what the voltage divider does. Assuming that you don't need to have a lower reference voltage than 5V to get the accuracy and range needed for any other use of the ADC, just using VCC as the ADC reference works.
The "secret voltmeter" is using on on chip 'band gap' source, which as noted, isn't that accurate, but can be calibrated. Depending on how accurate you need the measurement, that calibration may need to take into account the temperature and (uh oh), the VCC voltage. But, it's actually all calibratable if you need to do that. You are measuring what is sort of a known value (1.1V) with an unknown voltage reference, and back calculating what the voltage reference (which is VCC) is.
The only thing the divider can do is lower the voltage of the reference. If you get it closer to the 1.1v of the band gap, you get more accuracy in the reading. I don't think you need that.
I'm doing the same thing, although I need to use the AREF because I really do need to get more accuracy at lower voltages from the ADC. My project involves measuring the resistance of a cable, to around 1/4 of an ohm. With a 100 ohm pullup resistor, the cable and the resistor make a voltage divider, and I'm measuring the voltage to calculate the resistance. I don't need more than 100 ohms max, so I'm using the divider to get a 2:1 reduction in the AREF, which gives me one bit more resolution in the measurement. With 2.5V full scale, 100 ohms in 10 bits is tenths of an ohm, but noise and other problems make that not realistic. Seems to work okay, but calculating the actual resistance only is possible if I know the VCC. The secret voltmeter is the way to find that out.
You might consider that powering the arduino from the dc jack uses the on board power regulator, which will give you tighter tolerance on the VCC than powering from USB will (usually). My project has to be battery powered, and I'm not going to be able to regulate the output of the battery, but you should be able to use the DC jack.