What does it mean when an iphone 4s is locked to an AT&T network?

What do people mean when an iphone 4s is locked to the ATT network and how can you unlock it?

Depending on which service you're using, the phone is set for just that carrier. That means that though the phone itself isn't really different, if you go to Verizon or Sprint, it won't work. Apple sells iPhones that are unlocked and can be used on other GSM networks (Sprint and Verizon are not GSM), but they're roughly 3 times as much as the locked phones. There are ways to unlock the phones without buying one for the higher rate, but you'll need an Apple geek to show you how. The problem is that it may cancel any warranty on the phone.

It means that you can't go use it with someone else like Virgin Mobile.
You can obtain an unlocked iPhone 4S either by buying one directly from Apple (Extremely Expensive) or by using cracking applications

It means you can only use it with AT&T. No other network.
If you unlock it, you can use it with any. :3

It means that you can only use att with the iphone.