Site navigation refers to the buttons, links (and even the site architecture) which allows a new visitor to quickly get around and explore your site.
Main navigation buttons and links must be consistently placed on each page so the visitor does not have to lose their train of thought in order to continue to their next topic of interest. Text labels must be clear and unambiguous.
For very large sites, the site architecture also comes into play. A site may have so many pages that it is inconvenient (and messy) to place buttons for every page - on every page of the site. The most common architectures are hierarchical or radial, or a combination of the two.
Good Structure + Good Button Layouts = Good Results.