What is "verify if the latest Android firmware is available for your device"?

[piero]

it means are you up to date regarding stock OS ?

@piero

what you must take care about

Your Stock OS Android version, must be the latest available relative to the /e/OS build you want to install → not higher !

Remove all Gogol accounts, screen lock or SIM lock from the device.

@piero

If you plan to relock the bootloader after the installlation

this is not mandatoty, and only available with Official version, why relocking ?

Most important thing to take care about is the “android Security Update" aka "Security Patch Level” date (SPL) included in the already installed Operating System on your device at time of the unlock.

It must be OLDER than the “android Security Update" aka "Security Patch Level” date (SPL) included in the /e/OS build you want to install, otherwise you risk to change your device into Brick.

• You will find this information about stock OS on the device :

settings --> about phone --> android version 15 --> android security update

• you will find this information about /e/OS build on the release article about the /e/OS version you want to install :
  the title of the article need to match both “/e/OS version” and “android level”,
  currently "e-3.2-a15"

Releases · e / os / Releases · GitLab

more detailled explications here : (answered) Install /e/ on FP5 with newer Security Patches