Squid Proxy server is generally used in Unix-like platforms that are used to reduce the traffic and lower the bandwidth from server to client. A squid server that stores the frequently requested data from the client and stores that data in the form of cache. It can also be used for security purposes.
For more explanation on this video: https://www.linuxhelp.com/how-to-install-and-configure-squid-proxy-server-on-centos-7