Macworld A SIM (Subscriber Identity Module) is a tiny bit of circuitry that uniquely identifies your cellular modem in a phone, tablet, or other device to nearby cellular networks to allow access for voice, data, and texting, and manage billing, whether on your home network or a roaming one. We’ve used SIM cards for decades, even as they have shrunk from full size to micro to nano. The introduction of eSIM eliminates an external card entirely: a reprogrammable SIM circuit (or more than one) is stored within a device with a cellular modem, letting you set up or change service with a downloadabl…