what is _ENV in lua? and how can it be used?

Hi, so i've been tinkling around with all that lua has to offer, and i've learned a lot.... but I just don't I understand _ENV, all the explainations of it don't really make much sense to me..

i've glimpsed on some text that says, 'it stores global variables.' but when i do

print(_ENV)

it prints nil

which left me to conclude that, if it's nil then it must not be a part of the language, but i don't have 100% faith in my conclution, so I came here..

Thank you!