hoonami

I know this was discussed many many times but I really didn't understand everything about programing them. I bought a used 02 ES300 and it came with only one Master Key(Key with Remote in-one). And I just ordered a OEM Master Key(where you give them your VIN# and they cut it for you). I know when I get em I have to program the Remote Part which I've done before. But I don't know what Immobilizer Programming does... can someone clear this up for me???