An environment file is a script that:
Loads software libraries into your login environment. This is often done with a module manager such as lmod, spack, or environment-modules.
Stores settings for GEOS-Chem and its dependent libraries in shell variables called environment variables.
You will source the environment file each time you log in with a command such as:
$ . ~/my-environment-file # or whatever you name it
For general information about how libraries are loaded, see Load software into your environment.