An embedded SIM, or eSIM, is a digital SIM card permanently soldered onto a phone's motherboard. Unlike a physical SIM (a plastic card with a chip), an eSIM is provisioned remotely by a carrier — the cellular plan is delivered to the phone over the air or via QR code, then installed as software.
Modern phones can store multiple eSIM profiles and run two simultaneously. iPhone XS and newer support 8 stored profiles with 2 active; iPhone 14 and newer in the US are eSIM-only (no physical SIM tray). Pixel 3+ and Galaxy S20+ also support eSIM.
For business phone use cases, eSIM lets a carrier provision a real cellular line onto a customer's existing phone — without sending a physical card and without taking over the customer's primary number. That's the technical unlock behind business-eSIM products like Kelir.