It's fast_cgi cache from nginx to never call the slow php web site. Just it cache the first time the returned page and return always the same. Useful for 95% of content because without login it never change.
It's total purge each time at 0h0min, and nginx don't keep more than 24h cache on 200 (normal page), 301/302 redirection, 404 (not found).
You can manually delete it via:
rm -Rf /tmp/nginx/*
It's enabled by default util you do POST request. Done via login, register, add to card into shop. When you do that, it do CDIS cookie and disable the cache.
This system protect against the basic DDOS attacks, in 2017 we fight against 95% of basic DDOS attacks.