APN (Access Point Name) is the configuration that tells your phone which server to use for internet access via mobile networks - like an address for your internet gateway.
Phones from major carriers come pre-configured. However, switching to a budget MVNO SIM often requires manual APN setup.
APN details (name, username, password, authentication type) come with your SIM documentation. iPhones often just need to download and install an "APN configuration profile."
Wrong APN settings mean no internet outside Wi-Fi. If your new SIM can't connect, check APN settings first. Your MVNO's support page has step-by-step instructions.