A QR code looks like a link, and it is easy to treat it as one. It is closer to a key. It carries the details a phone needs to fetch one particular plan from one particular server, and using it is the installation rather than a step towards it.
That is why it is single use, and why the small practical decisions around it matter more than they seem to.
What the code actually contains
An eSIM activation code has three parts: a format marker, the address of the server that holds the plan, and a matching identifier that points at your plan on that server. The QR code is those three parts written in a form a camera can read.
Nothing in it is a password in the usual sense. It is a claim ticket. Anyone holding it can claim the plan, which is the reason for every piece of advice below.
Where it arrives
Two places, and you should make sure both are usable before you travel.
The first is the screen, at the moment of purchase. The second is email, and the email is the one that matters later, because it is where the server address and activation code live in text form.
Save both to the phone before you go. A screenshot of the QR code, kept in your photos, works when the email will not load. What does not work is arriving at an airport with no data, an email app that needs a connection, and a code you have never seen.
How to scan it
The installation lives in the phone’s settings, not in the camera app, and this is the step people get wrong.
On an iPhone, open Settings, then the mobile data section, and look for the option to add an eSIM or cellular plan. On Android the path is similar: Settings, then the network or connections section, then SIMs, then add an eSIM. Either way the phone offers to scan a code or to enter the details by hand.
Point the camera at the code and wait. The phone reads it, contacts the server, and installs the profile. You will be asked to name the new line, which is worth doing properly, because a line called Travel is easier to find at midnight than one called Secondary.
When scanning does not work
Three things go wrong most often, and none of them means the plan is lost.
The first is a dirty or badly lit code. A code on a screen with the brightness turned down, or a printout that has been folded, can be unreadable. Turn the brightness up, or hold the phone further away.
The second is a phone that will not accept the code at all, usually because it is locked to a network. That is a phone problem, not a code problem, and it cannot be fixed by scanning harder.
The third is the code having already been used. If you scanned it once and the install failed halfway, the plan may already be claimed. Contact the company that sold it, because they can see the state of the plan and can usually issue it again.
If the camera simply will not cooperate, type the details in. Every eSIM comes with a server address and an activation code next to the QR code, and the manual option takes both. It is the same installation by a slower route.
Why you should not share it
Do not post a screenshot of your QR code in a group chat asking whether it looks right. Do not send it to a friend so they can read it out to you. Do not photograph it and leave it in a shared album.
The reason is that the first phone to install it gets the plan. There is no second factor and no confirmation step. Someone with the code and an unlocked phone can take the plan before you do.
The same applies to the text version. If you post the server address and matching identifier anywhere public, you have posted the plan.
Keeping the code afterwards
Once the plan is installed and working, the code has done its job. Keep the email anyway, because it is the record of what you bought and when, and it is what a support conversation will ask for.
Delete nothing until the trip is over and you have checked that the plan finished as expected. A code that has been installed cannot be reused, but an order email is still evidence, and it is much easier to resolve a problem with it in hand.
If you would rather understand the technology before you buy, what an eSIM is covers it from the beginning, and eSIM against a physical card deals with the choice between the two.