The eSIM is a SIM card which is integrated in a mobile phone and possible to connect to any operator offering eSIM services.
Basically the eSIM functions same as a traditional SIM card, but the physical SIM card not needed to use it. It is actually pre-installed in the device and you can activate it by installing the “eSIM profile” of a any operator.
An example is like the jiophone. They have eSIM in their jiophone.
Actually, it is the electronic copy of the physical SIM card. There are many smartphones, tablets and laptops are equipped with an eSIM card.