Varnish is a high-performance HTTP accelerator.

Varnish is an HTTP accelerator. An HTTP accelerator (often called Reverse
Proxy) is an application that stores (caches) documents that have been
requested over the HTTP protocol.

Based on certain criteria the next client requesting the document is either
given the cached document, or a "fresh" document requested from a backend
server. The purpose of this is to minimize the requests going to the backend
server(s) by serving the same document to potentially many users.