Pay as you go SIM is a service that activates a subscriber identity module

Subscriber Identity Module

A subscriber identity module or subscriber identification module is an integrated circuit that securely stores the International Mobile Subscriber Identity and the related key used to identify and authenticate subscriber on mobile telephony devices .A SIM is held on a removable SIM card, which...

 (SIM), most commonly used to identify a subscriber on mobile phone network, for a period determined by the credit remaining on the SIM. Access and use of a mobile phone network is usually obtained by paying a monthly subscription for unlimited (or virtually unlimited) access during the subscription period. Pay as you go is limited instead by the unused credit remaining on the network.