Varnish is a program that can greatly speed up a Web site while reducing the load on the Web server. According to Varnish’s official site, Varnish is a “Web application accelerator also known as a caching HTTP reverse proxy”.
When you think about what a Web server does at a high level, it receives HTTP requests and returns HTTP responses. In a perfect world, the server would return a response immediately without having to do any real work. In the real world, however, the server may have to do quite a bit of work before returning a response to the client.