Your site probably displays a lot of information that should not be indexed, such as your menu structure or footer. Or maybe entire pages do not need to be indexed.
In the ignore_content_by_css_selector key of the site-search config file, you can specify CSS selectors of elements that should not be indexed. By default, the content of a nav element will not be put in the index (but all URLs inside it will still be crawled).
Additionally, any elements with a data-no-index will not get indexed. In the following example, the sentence "This will not be indexed", will not be indexed.
<html>
<head>
<title>This is my page</title>
<metaname="description" content="This is the description">
</head>
<body>
<nav>
This is the content of the nav. It should not be indexed.
</nav>
<h1>This is the H1</h1>
This is the content
<div data-no-index>
This will not be indexed.
</div>
</body>
</html>
In the ignore_content_on_urls key of the site-search config file, you may specify URLs whose content should not be indexed. All links on these URLs will still be followed and crawled.
If your site's response contains a header whose name is in the do_not_index_content_headers of the site-search config file, then that page will not be indexed.