They are talking about Varnish: https://www.varnish-cache.org https://www.varnish-cache.org Or, you can start with this to get the general idea first: https:

They are talking about Varnish: https://www.varnish-cache.org https://www.varnish-cache.org Or, you can start with this to get the general idea first: https://en.wikipedia.org/wiki/Varnish_software https://en.wikipedia.org/wiki/Varnish_software