Network Locked means you can't move it to another network.
Why? For example: if an AT&T is under an installment plan that someone stopped paying on, it would be network locked (to AT&T) but might otherwise be activate-able with what looks like a clean imei. For AT&T phones you can check locked status online from their website with their Unlock Device Portal as long as you have the imei of the phone. It will tell you the reason why it can't be unlocked if it can't.