Environment variables are how the outside world hands configuration to a
program without changing its code. A container runtime sets PORT, a CI
system sets CI=true, a deploy pipeline sets DATABASE_URL. Your Go
program reads them at startup and adjusts its behaviour.
The os package is the door to all of this. Everything a Go program needs
to read, set, or list environment variables lives there, so any file that
touches the environment starts with an import "os".
The next few nodes each take one function from that package: os.Getenv
to read a value, os.LookupEnv to tell unset from empty, os.Setenv to
set one, and os.Environ to list them all.