AT&T SIM card?

What exactly does this do? I bought a iPhone from someone who is from the US but then I locked it.​

 

SIM card is short for Subscriber Identity Module card. What it does is basicly hold information about personal numbers on the phone, security information, subscriber information, cell phone number, text messages, etc. When your SIM card is locked then that basically means that you can't put your SIM card in different carriers phone. EX: a locked AT&T SIM card will not work with a Verizon phone. I hope this helps.

 

The sim card simply gives your iPhone service.
If the iPhone was sold in the US its going to locked. The only way to get it unlocked to another Carrier would be to "Jailbreak" the phone, which is over-writing the iPhones Software version with a Modified IPSW file, which is the Iphones software.

If you do some research About Jailbreaking your iPhones Model and Software version, you can find your Software version in Settings>General>About.

 

Get it unlocked at a @t@t store....i got my sim card locked and a guy at AT@T got it unlocked for me...erased all my contacts though.

 

You're generally suppose to tell the service provider to save your contacts before get your SIM unlocked.