Network lock means the handset is locked to a particular mobile provider. You'll need to get in contact with the person your purchase the phone off and ask them where they purchased the phone from. Once you know who the provider is you'll be able to go to the providers website to unlock the phone. Our one for example is http://www.vodafone.co.nz/help/mobile-phones/unlock/
PUK code is a PIN configured on the SIM card for steps on how to locate this click here.