Three letters that have derailed more holiday mornings than lost luggage: APN. It stands for Access Point Name, and it is, despite the acronym-soup, one of the least mysterious things in your phone - once someone actually explains what it's for instead of just telling you to "check it."
What an APN actually does
Every mobile network runs on infrastructure, and an APN is the address your phone uses to find the right gateway into it - think of it as your phone asking "which door do I use to get onto the internet through this network." Get the address wrong, or don't have one at all, and your phone can still show full signal bars while sending zero data anywhere, which is exactly the confusing state that sends people searching for "APN settings" at 2am in a hotel lobby.
The reassuring part: for the overwhelming majority of eSIM installs, you never touch this. When your phone downloads a travel eSIM profile, the correct APN comes bundled with it automatically, the same way your home network's APN was already configured the day you got your current phone. You're not meant to know it exists, in the same way you're not meant to know your electricity company's substation address.
A quick check before you touch anything
APN problems get blamed for a lot of symptoms that aren't actually APN problems. Before you go anywhere near this menu, rule out the easier causes first: is Airplane Mode definitely off, is the travel eSIM actually selected as your data line, and is Data Roaming switched on for it. Those three settings cause the overwhelming majority of "no data" reports, and none of them involve typing anything into a settings field.
When it actually comes up
Occasionally - and it is genuinely occasional - a phone doesn't pick up the automatic APN cleanly, usually after a manual network reset, a very old Android build, or a device that's had several SIMs cycle through it in a short space of time. The symptom is specific: full signal, mobile data switched on, and still nothing loads.