An eSIM is a SIM card that already lives inside your phone, soldered to the board. Instead of slotting in a piece of plastic, you download a carrier profile onto the chip that is already there. The "e" stands for embedded.
In practice it changes one thing, but that one thing changes everything: you can buy a data line for another country from your sofa and have it ready before you leave home — no shop, no waiting for post.
There are two conditions, and it is only fair to say them before you buy: your phone must support eSIM and must not be locked to a carrier. And unlike a plastic card, an installed eSIM does not move to another phone — it stays tied to that device.
After buying an eSIM, you get an activation QR code. Installing it is quick and you don't need to swap your main SIM — the eSIM works as a second, data-only line for your destination.
You can install the eSIM before traveling — data only starts counting when you connect to the network at your destination. That way you arrive with internet ready to use.