The original network is the carrier that sold the phone (either to you or the original owner) and therefore responsible for configuring the device so that you can only use it with one of their SIM cards. It will almost certainly be the only network you can use the phone on while locked.
If you do not know the original network, in some cases, you have the option to select “Network Operator Unknown or Not in List" on the online order form.
If this option isn’t available for your phone, we recommend using our Original Operator Check before submitting your unlocking request: