Site Search is the built in Sitemason feature that allows the viewers of your website to search the content on your site. This isn't to be confused with searching in the Sitemason interface which is an administrative tool for you to quickly find and edit your own content.
Setting up the Site Search in your website is likely to be done by your web designer, but you do have control over which pages are indexed and available in search results to your site visitors.
In each page's Settings Panel, there is a single option to "Include in Site Search". By default this is selected, but by unchecking this option, the page will be omitted from Site Search. This might be useful for a page in who's content is still in draft, a private page, or perhaps duplicate content that might confuse the visitor.