How external power supply works???

Hi all! I know that probably this question might be dump but I've read tons of articles about external power supply and I still can't figure it out.

My question is this;For what reason is the Vin pin ? You put an external power supply or you get from the pin more volts?????
But in both cases there is a problem...If you put an external power supply to the pin from where you'll take the voltage?( i hope i am not too confusing)
If from the other side take from that pin where do you put the externall power supply ???? (i hope i am not too confusing again :))

Please repply so that i will be able to understand 8)

The "Vin" pin is an alternative to the coaxial power connector on the Arduino The only difference is a diode in case you plug in a backwards-wired power supply to this connector.

Either you supply power via the USB connector, or via the coaxial connector, or you connect a positive supply of suitable voltage (7 to 15V) to Vin. If you are supplying it via the coaxial connector, it will be convenient to use Vin as a connection to take power (no more than 1 amp and depending also on your power pack) for other devices.