Squid is a Linux proxy server.
A proxy server generally is a server that saves the visited web pages for later requests, so if you try to visit the same web page or any one else, the page will be retrieved from the proxy server.
It makes web surfing much faster and reduces the traffic which means less cost.
Caching servers can decrease external traffic up to 45%.
Another main advantage for proxy servers, you can configure the proxy with some settings for access control. For example, you can restrict access to specific websites.
If you surf the web before from an anonymous proxy, this is actually a proxy server.