The short answer
An eSIM (embedded SIM) is a small chip soldered inside your phone that does the same job as a plastic SIM card. Instead of pushing a physical card into a tray, you download a SIM profile onto that chip, usually by scanning a QR code. Once installed, your phone connects to the mobile network exactly as it would with a normal SIM: calls, texts and data all work the same way.
What actually happens when you install one
Installing an eSIM means downloading a small carrier profile onto the chip in your phone. The QR code you scan is just an address: it tells your phone where to fetch the profile from (an SM-DP+ server, in the jargon) and includes a one-time activation code. That is why you need an internet connection for the install, and why a code typically works only once. After the download, the profile lives on your phone and no longer needs that link.
eSIM vs physical SIM: what changes
The network treats them the same; the difference is logistics. A physical SIM has to be produced, shipped or picked up, and swapped by hand. An eSIM is delivered by email in minutes, cannot be lost or damaged, and does not take up your SIM tray. The trade-off: an eSIM profile is tied to the phone it was installed on, so moving to a new phone usually means a new profile rather than moving a card across (a real difference worth knowing before you switch).
Why travellers use them
Because you can arrive already connected. With a travel eSIM you buy online before the trip, install it at home on Wi-Fi, and switch it on when you land, keeping your home SIM in place for calls and texts on your own number. No airport SIM counter, no passport registration for a local SIM, no juggling a tiny plastic card at the baggage belt. That convenience is the whole pitch: for very long stays or very heavy data use, a local SIM can still win on price.
What you need to use one
Two things: a phone with eSIM support, and a phone that is carrier-unlocked. Most phones from about 2019 on support eSIM (iPhone XS/XR and later, recent Samsung Galaxy models, Google Pixel 3 and later, and many other Android phones), but region variants differ, so it is worth checking your exact model. A quick test: dial *#06# and look for an EID number; if one appears, your phone has eSIM hardware. Locked phones may block other carriers' profiles, so confirm with your provider if you bought the phone on a contract.
Is an eSIM secure?
An eSIM is at least as secure as a plastic SIM, and in some ways more so. It cannot be pulled out of a stolen phone and used elsewhere, and profiles are delivered over an encrypted, standardised process (the same GSMA specification carriers worldwide use). The practical risks are the same as with any SIM: guard the QR code like a password, since anyone who installs it first uses up its one-time activation.
