An e-sim is a built-in SIM card
An e-sim (also called embedded-SIM) is a SIM card that is built into a device (phone, tablet) and which is remotely programmable. This makes it easy to change the provider without having to change a SIM card. It is also possible to use one subscription over multiple devices: So 1 subscription You use for your smartwatch, your tablet, and your smartphone. And if you want a different subscription then you simply move on.