The protocol that powers the internet and all methods within it. Whether you email, browse websites, stream a TV show, transfer files, SSH to a server or any and every other method of using the internet; it all works on top of the TCP/IP protocol.
Whilst understanding the fundamental aspects of how the internet works is important, detailed knowledge of TCP/IP isn't required to be an effective and knowledgeable technical SEO.
However, I do think a basic understanding is important and as such have included a basic overview below and a video explaining in more detail. There will not be a test on understanding this and so if you wish to simply go through this course to get a pass mark, then you can jump off and not dig deeper.
However...
I sincerely hope you want to be a well rounded and truly technical, Technical SEO. As such I believe you would be doing yourself a service if you did study what makes our internet tick and how the fundamental protocols work.
The choice is yours :)
From: https://en.wikipedia.org/wiki/Internet_protocol_suite
The Internet protocol suite is the computer networking model and set of communications protocols used on the Internet and similar computer networks. It is commonly known as TCP/IP, because its most commonly used protocols, the Transmission Control Protocol (TCP) and the Internet Protocol (IP) were the first networking protocols defined during its development. It is occasionally known as the Department of Defense (DoD) model, because the development of the networking model was funded by DARPA, an agency of the United States Department of Defense.
The Internet protocol suite provides end-to-end data communication specifying how data should be packetized, addressed, transmitted, routed and received. This functionality is organized into four abstraction layers which are used to sort all related protocols according to the scope of networking involved. From lowest to highest, the layers are the link layer, containing communication methods for data that remains within a single network segment (link); the internet layer, connecting independent networks, thus providing internetworking; the transport layer handling host-to-host communication; and the application layer, which provides process-to-process data exchange for applications.