An eSIM (embedded Subscriber Identity Module) is a digital version of a physical SIM card that is built directly into a device's motherboard. It allows users to connect to a mobile network for calls, texts, and data by downloading a network profile and activating it digitally, eliminating the need to physically swap cards. eSIMs offer flexibility, the ability to store multiple profiles for different carriers, and enhanced security, as they cannot be physically removed if the device is stolen.