Unlocking your phone means that you will be able to use the phone with carriers other than original.
By default Samsung Galaxy S 3 is locked to T-Mobile USA & if you try to use it with different carrier, phone will prompt you to enter unlock code or “Network Unlock Key”.
Unlock code for each device is unique & generated based on IMEI of this device.
To obtain IMEI you will need to dial *#06# or check behind the battery for 15 digit number. Sometimes it’s a 17 digit number with “.” or “/” symbols. But you just need to disregard other symbols & get first 15 digits – this would be IMEI of the device.
Here is the video of how you can obtain IMEI of your device.