eSIMs are basically digital versions of a physical SIM card. Instead of installing a physical SIM card, carriers can set up an eSIM on your phone with your phone number remotely, without a trip to the carrier store, or waiting for a physical SIM in the mail.  You can add multiple eSIMs to your iPhone, which can be handy for travelling, as long as carriers abroad also support eSIMs. Sometimes, a physical SIM is more compatible and easier to find. It depends on the carriers available in other countries.