A Guide is a functional description of how something in Max works. Guides are like manuals for specific aspects of Max. Every area of the software (Max, MSP, Jitter, Gen, etc.) has it's own set of Guides. Have you ever wondered what Snippets are? That paperclip icon on the left side of a patcher. Go to Max > Guides > Snippets to learn more. Or are you curious what the difference is between MC and MCS objects? Go to MC > Guides > MC vs MCS Objects for an in depth explanation.
To access Guides, navigate to the Documentation home page, expand the section (Max, MSP, Jitter, etc.) you want to learn about, then click on "Guides".