An activation code is the string of text that tells your phone where to fetch its eSIM profile and which one to grab. It is what sits behind the QR code you scan when you set up a travel eSIM. Read the QR with a magnifier and you would see something starting with LPA, which stands for Local Profile Assistant, the part of your phone that handles eSIM downloads. That prefix is why the string is often called an LPA string.
The code has two important pieces. The first is the SM-DP plus address: the server run to hold prepared eSIM profiles until a phone comes to collect one. The second is a matching ID that points to your specific profile on that server, so your phone downloads yours and nobody else’s. Put together, they read like a delivery address plus a parcel number. When you scan the QR, your phone reads both, connects to the server, and pulls down the eSIM profile over the internet.
The reason we still hand out the raw code alongside the picture is that a QR does not always scan. Maybe you are reading the code from the same screen you need to scan with, maybe the camera is fussy, maybe the image is a little blurry. In that case you switch to manual entry: on an iPhone you choose to enter the details by hand, then paste the SM-DP plus address and the ID into their fields, or paste the whole activation string into one box on newer versions. The result is identical to scanning; you have just typed the address instead of photographing it.
If you get stuck, our install and activate guide walks through both the scan path and the manual path screen by screen. Keep the activation code somewhere handy until the profile is installed and showing signal, since you may need it again if you reset the phone before your trip.