Both can also be found along the top bar when you're logged into SWC, along with settings, live chat link, and so on.
The guide is an OOC knowledge base that helps you understand game mechanics and how things work. For instance, you can find articles on how hyperspace works, as well as concepts like hunting, loot drops, and mining.
The Holocron is an IC (In-character) wiki that holds player-created pages on things such as individual characters and significant events. You don't need to visit a specific area in-game to access either of these, they can be accessed at any time, from any location.
Hope this helps, and if you have further questions feel free to reach out!