Your mobile number is precious. It's the phone number everybody knows. So take it with you when you change network service providers.
Port Authorisation Code or PAC for short, is a code generated by your old phone network to authorize your new mobile provider to port your number over to them. It is free and valid for 30 days.
Note: You can transfer your number even after you purchase your new mobile contract or phone SIM. It is not necessary to have your PAC number before buying your new mobile package.
Need some help? Ask over at the new Mobile Phone Forum