What are proxies?

I had a look on the Internet and couldn't find a clear definition, could somebody explain why you would need proxies on an instagram bit aswell? Thanks

Well you need proxy to let your computer obtain IP As same as the region you posting your ads too, or changing Ips to create eMails etc.
But here is more Info.

A proxy server is a computer that offers a computer network service to allow clients to make indirect network connections to other network services. A client connects to the proxy server, then requests a connection, file, or other resource available on a different server. The proxy provides the resource either by connecting to the specified server or by serving it from a cache. In some cases, the proxy may alter the client's request or the server's response for various purposes.

More Info...
http://whatismyipaddress.com/proxy-server

Regards

In computer networks, a proxy server is a server (a computer system or an application) that acts as an intermediary for requests from clients seeking resources from other servers. A client connects to the proxy server, requesting some service, such as a file, connection, web page, or other resource available from a different server and the proxy server evaluates the request as a way to simplify and control its complexity. Today, most proxies are web proxies, facilitating access to content on the World Wide Web.

what is a private proxy and what is a proxy
Instagram proxies - You need them if you want to use multiple Instagram accounts or for Marketing purpose !

There are also different kind of proxies (performing via different protocols). Http proxy, Socks5 and Socks4.

Best choice to effectively hide your IP would be Socks5 proxy from my experience.