The functions.php file sets up the theme and functions for the theme. The Starkers fuctions.php file is the same as the one for Twentyten, except for a section added in the middle about the title. Not sure why it is in Starkers when it is not in Twentyten, since Starkers is derived from Twentyten.
Following is a description of the contents of the functions.php file in the theme:
The functions php file starts off by setting the content width based on the theme’s design and stylesheet. It then sets up theme defaults and registers support for various WordPress features. Next is a section which styles the header iamge displayed on the Appearance > Header admin panel.
The next section, contained in Starkers only, not in Twentyten, makes some changes to the <title> tag, by filtering the output of wp_title(). Following this are sections on getting the wp_nav_menu() fallback, wp_page_menu() to show a home link, setting the length of post excerpts to 40 characters, returning a “Continue Reading” link for excerpts, adding a pretty “Continue Reading” link to custom post excerpts, removing inline styles printed when the gallery shortcode is used, and setting up templates for comments and pingbacks.
The next section registers widgetized ares, including two sidebars and four widget-ready columns in the footer. The default styles are removed that are packaged with the Recent Comments widget. Then it prints HTML for the meta information for the current post-date/time and author and the current post (category, tags, and permalink).