Nova Lut ENV???

So I'm just getting into modding and have been installing and checking out some cool mods. But in my naive experience there are some terms that I just don't understand and cannot find any info on. Specifically, with the Nova Lut mod. The description says to choose an ENV. But for a smooth brain like me I have no idea what that is. I tried searching on reddit and Nexus mod forums but I cannot find any explanation of what the ENV is. Is it another mod? Searching it doesn't turn up anything. Can anyone point me in the right direction?